Fix drag-reschedule issues found in review (#68)

Write path:

- realignRecurrence now handles weekly BYDAY only. What the rule must agree
  with is the series *anchor*, which moves by the same wall-clock shift as the
  dragged occurrence — weekday survives that (uniform mod 7), day-of-month does
  not. BYMONTHDAY=28 with a January anchor, occurrence Feb 28 dragged to Mar 1,
  produced a Jan 29 anchor under a BYMONTHDAY=1 rule: a DTSTART that is not an
  instance of its own rule. Also refuse when the shift is not a whole number of
  days, which would carry a late-enough anchor across an extra midnight.
- The accepted parts are now a subset of parseSimpleRecurrence's, so anything
  realignable is a rule the UNTIL guard can actually read. FREQ=MONTHLY;BYDAY=MO
  previously slipped past it and could move a series past its own end.
- The UNTIL check moved to write time and picks the date the chosen scope
  actually starts at: the anchor for a whole-series move, the occurrence for a
  split, nothing for a single occurrence. It used to test the occurrence in
  every case and refused the ordinary drag of a bounded series' last one.
- shiftedByDays preserves the instant duration for timed events, as shiftedTo
  already did — a month drag onto a DST changeover rewrote the whole series'
  DURATION.
- Guard against a second drop landing while one is in flight; the shifts would
  compound on the re-read anchor.

Gestures:

- A long press that never moved wrote a reschedule: the target snaps to the
  15-minute grid at pickup, so an event at 09:07 resolved to 09:00 the instant
  it lifted. The drop is now refused when it lands on the slot it started from.
- After pickup the gesture is driven and consumed on the initial pass. Pinching
  mid-drag used to zoom and drag at once, then write wherever the zoom left the
  block; the month grid's tap layer opened the day on lift.
- RTL: pointer x was mapped to columns as if the grid were LTR, so a drop landed
  6-n columns away, and both ghosts used the direction-aware offset with root
  coordinates, mirroring them across the screen.
- The month drop passes the day delta instead of a date, so the grid and the
  view model no longer each resolve the event's first day in their own zone.

app/src/main/java/de/jeanlucmakiola/calendula/domain/EventShift.kt

@@ -40,13 +40,24 @@ fun EventForm.shiftedTo(newStart: Instant, deviceZone: TimeZone): EventForm {
 
 /**
  * The form moved [days] calendar days, keeping its time of day and its length.
- * Both ends move, so a multi-day event keeps its span rather than collapsing to
+ * A multi-day event keeps its span rather than collapsing to a single day.
- * a single day; an all-day event's placeholder times ride along untouched.
+ *
+ * An all-day event is pure date arithmetic — both ends move, and the placeholder
+ * times ride along untouched. A timed one re-derives its end from the preserved
+ * **instant** duration, for the same reason [shiftedTo] does: the length travels
+ * to a recurring row as `DURATION`, so a move onto a DST changeover that kept
+ * wall clock would silently rewrite the whole series' length.
  */
-fun EventForm.shiftedByDays(days: Int): EventForm {
+fun EventForm.shiftedByDays(days: Int, deviceZone: TimeZone): EventForm {
     if (days == 0) return this
+    val newStart = LocalDateTime(start.date.plus(days, DateTimeUnit.DAY), start.time)
+    if (isAllDay) {
         return copy(
-        start = LocalDateTime(start.date.plus(days, DateTimeUnit.DAY), start.time),
+            start = newStart,
             end = LocalDateTime(end.date.plus(days, DateTimeUnit.DAY), end.time),
         )
+    }
+    val zone = resolvedZone(deviceZone)
+    val span = end.toInstant(zone) - start.toInstant(zone)
+    return copy(start = newStart, end = (newStart.toInstant(zone) + span).toLocalDateTime(zone))
 }

app/src/main/java/de/jeanlucmakiola/calendula/domain/RecurrenceRealign.kt

@@ -1,7 +1,6 @@
 package de.jeanlucmakiola.calendula.domain
 
 import kotlinx.datetime.LocalDate
-import kotlinx.datetime.number
 
 /**
  * [rrule] re-anchored from an occurrence on [oldStart] to one on [newStart].
@@ -9,41 +8,55 @@ import kotlinx.datetime.number
  * `Events.RRULE` is written verbatim while DTSTART moves, so a rule that names
  * its own day — `FREQ=WEEKLY;BYDAY=MO`, what Google and CalDAV write for nearly
  * every weekly series — would keep pointing at Monday after the anchor became a
- * Wednesday, and the series would not move at all. The day-selecting parts are
+ * Wednesday, and the series would not move at all. `BYDAY` is therefore
- * therefore re-derived from [newStart].
+ * re-derived from [newStart].
  *
- * Returns null when the rule picks days in a way one moved occurrence cannot
+ * **Only weekly `BYDAY` is realigned, and only for a whole-day move.** The rule
- * resolve (`BYDAY=MO,WE`, an ordinal `2TH`, `BYSETPOS`, …). There is no safe
+ * has to agree with the series *anchor*, which is not the occurrence being
- * guess there — dropping the extra days would delete occurrences — so the caller
+ * dragged: the anchor moves by the same wall-clock shift, and only weekday
- * refuses to move anything but that single occurrence.
+ * arithmetic survives that unchanged, because it is uniform mod 7 and every
+ * anchor time-of-day crosses the same number of midnights. Day-of-month does not
+ * — `BYMONTHDAY=28` on a January anchor dragged from Feb 28 to Mar 1 gives an
+ * anchor of Jan 29 under a `BYMONTHDAY=1` rule, which is not an instance of its
+ * own rule and materialises a phantom occurrence on any client that trusts
+ * DTSTART. Those rules return null instead. The caller must also refuse when the
+ * shift is not a whole number of days, for the same reason (see
+ * `RescheduleViewModel`).
+ *
+ * Null also covers rules one moved occurrence cannot resolve at all
+ * (`BYDAY=MO,WE`, an ordinal `2TH`, `BYSETPOS`, …) — dropping the extra days
+ * would delete occurrences. The accepted parts are deliberately a subset of what
+ * [parseSimpleRecurrence] understands, so anything realignable is also a rule
+ * [problems] can check the `UNTIL` of.
  */
 fun realignRecurrence(rrule: String, oldStart: LocalDate, newStart: LocalDate): String? {
     if (oldStart == newStart) return rrule
     val prefix = if (rrule.startsWith("RRULE:")) "RRULE:" else ""
     val parts = rrule.removePrefix("RRULE:").split(';').filter { it.isNotBlank() }
     if (parts.isEmpty()) return null
+    var weekly = false
     val rebuilt = parts.map { part ->
         val eq = part.indexOf('=')
         if (eq <= 0) return null
         val key = part.substring(0, eq).uppercase()
         val value = part.substring(eq + 1).trim()
         when (key) {
+            "FREQ" -> {
+                weekly = value.equals("WEEKLY", ignoreCase = true)
+                part
+            }
             "BYDAY" -> {
                 val old = RRULE_DAY_CODES[oldStart.dayOfWeek] ?: return null
                 if (!value.equals(old, ignoreCase = true)) return null
                 "BYDAY=${RRULE_DAY_CODES.getValue(newStart.dayOfWeek)}"
             }
-            "BYMONTHDAY" -> {
+            "INTERVAL", "COUNT", "UNTIL", "WKST" -> part
-                if (value != oldStart.day.toString()) return null
-                "BYMONTHDAY=${newStart.day}"
-            }
-            "BYMONTH" -> {
-                if (value != oldStart.month.number.toString()) return null
-                "BYMONTH=${newStart.month.number}"
-            }
-            "FREQ", "INTERVAL", "COUNT", "UNTIL", "WKST" -> part
             else -> return null
         }
     }
+    // BYDAY is only simple on a weekly rule (matching parseSimpleRecurrence);
+    // "every Monday of the month" is a shape this has not been reasoned about.
+    if (!weekly && parts.any { it.substringBefore('=').trim().uppercase() == "BYDAY" }) return null
+    if (parts.none { it.substringBefore('=').trim().uppercase() == "FREQ" }) return null
     return prefix + rebuilt.joinToString(";")
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/EventDrag.kt

@@ -121,15 +121,25 @@ fun rememberDragSurface(
                 if (!took) return@awaitEachGesture
                 var dropped = false
                 try {
+                    // Once the block is lifted the gesture is ours, so it is
+                    // driven on the initial pass and consumed there. That pass
+                    // runs parent → child, which gets both halves right: an
+                    // ancestor that outranks us (the pinch, which claims the
+                    // moment a second finger lands anywhere in the timeline) has
+                    // already consumed by the time we look, and everything below
+                    // us — the month grid's full-bleed tap layer, which is a
+                    // descendant and would otherwise open the day on lift — sees
+                    // ours. Consumption persists across passes, so the scroll and
+                    // the page swipe stand down on main as well.
                     while (true) {
-                        val event = awaitPointerEvent()
+                        val event = awaitPointerEvent(PointerEventPass.Initial)
                         val change = event.changes.firstOrNull { it.id == down.id } ?: break
+                        if (change.isConsumed) break
                         if (!change.pressed) {
+                            change.consume()
                             dropped = true
                             break
                         }
-                        // A second finger is the pinch taking over; it consumes on
-                        // the initial pass, so the drag is already dead.
                         if (event.changes.count { it.pressed } > 1) break
                         change.consume()
                         coordinates[0]?.takeIf { it.isAttached }

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/EventMoveHost.kt

@@ -55,7 +55,7 @@ fun EventMoveHost(viewModel: RescheduleViewModel, modifier: Modifier = Modifier)
 
     val moved = outcome as? MoveOutcome.Moved
     val movedLabel = moved?.let { formatMovedTo(it.startMillis, it.isAllDay, use24Hour, locale) }
-    val message = when (val o = outcome) {
+    val message = when (outcome) {
         null -> null
         is MoveOutcome.Moved -> stringResource(R.string.event_move_done, movedLabel.orEmpty())
         MoveOutcome.Undone -> stringResource(R.string.event_move_undone)

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/RescheduleViewModel.kt

@@ -6,10 +6,13 @@ import dagger.hilt.android.lifecycle.HiltViewModel
 import de.jeanlucmakiola.calendula.data.calendar.CalendarRepository
 import de.jeanlucmakiola.calendula.data.calendar.NoSuchEventException
 import de.jeanlucmakiola.calendula.data.di.IoDispatcher
+import de.jeanlucmakiola.calendula.domain.EventDetail
 import de.jeanlucmakiola.calendula.domain.EventForm
 import de.jeanlucmakiola.calendula.domain.EventFormProblem
+import de.jeanlucmakiola.calendula.domain.RecurrenceEnd
 import de.jeanlucmakiola.calendula.domain.RecurringWriteScope
 import de.jeanlucmakiola.calendula.domain.allowsEventMove
+import de.jeanlucmakiola.calendula.domain.parseSimpleRecurrence
 import de.jeanlucmakiola.calendula.domain.problems
 import de.jeanlucmakiola.calendula.domain.realignRecurrence
 import de.jeanlucmakiola.calendula.domain.resolvedZone
@@ -26,9 +29,11 @@ import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
+import kotlinx.datetime.DateTimeUnit
 import kotlinx.datetime.LocalDate
 import kotlinx.datetime.TimeZone
 import kotlinx.datetime.atStartOfDayIn
+import kotlinx.datetime.plus
 import kotlinx.datetime.toInstant
 import kotlinx.datetime.toLocalDateTime
 import kotlin.coroutines.cancellation.CancellationException
@@ -40,8 +45,14 @@ sealed interface MoveTarget {
     /** A new start instant — a timeline drag, which moves time and day at once. */
     data class Start(val instant: Instant) : MoveTarget
 
-    /** A new first day, keeping the time of day — a month-grid or all-day drag. */
+    /**
-    data class Day(val date: LocalDate) : MoveTarget
+     * A whole-day shift, keeping the time of day — a month-grid or all-day drag.
+     * A delta rather than a target date on purpose: the grid already knows how
+     * many columns the finger crossed, and re-deriving that from a date would
+     * mean the screen and this view model each resolving the event's first day
+     * in their own zone, which can disagree by a day.
+     */
+    data class ByDays(val days: Int) : MoveTarget
 }
 
 /**
@@ -122,6 +133,14 @@ class RescheduleViewModel @Inject constructor(
 
     private var pending: PreparedMove? = null
 
+    /**
+     * Set from the moment a drop is accepted until its write settles. Two drops
+     * of the same recurring event landing inside that window would each compute
+     * their shift from the same pre-move occurrence, and the data layer applies
+     * both to the re-read anchor — so the shifts would compound.
+     */
+    private var busy = false
+
     /**
      * The calendars whose events may be dragged. Nothing below the UI guards
      * this — the repository and data source attempt any write handed to them —
@@ -149,15 +168,24 @@ class RescheduleViewModel @Inject constructor(
          * wider than the single occurrence would leave rule and anchor disagreeing.
          */
         val canRealign: Boolean,
+        /** The series row's DTSTART date after the move — what its `UNTIL` must clear. */
+        val newAnchorDate: LocalDate,
     )
 
     fun move(request: MoveRequest) {
-        if (_scopePrompt.value != null) return
+        if (busy || _scopePrompt.value != null) return
+        busy = true
         viewModelScope.launch {
-            val prepared = prepare(request) ?: return@launch
+            val prepared = prepare(request)
+            if (prepared == null) {
+                busy = false
+                return@launch
+            }
             if (!prepared.isRecurring) {
                 write(prepared, RecurringWriteScope.AllEvents)
+                busy = false
             } else {
+                // Still busy: the scope dialog is now the thing in flight.
                 pending = prepared
                 _scopePrompt.value = MoveScopePrompt(occurrenceOnly = !prepared.canRealign)
             }
@@ -167,19 +195,28 @@ class RescheduleViewModel @Inject constructor(
     /** Answer the scope dialog. */
     fun moveWithScope(scope: RecurringWriteScope) {
         val prepared = pending ?: return
+        // Belt and braces against the dialog ever offering a scope the rule
+        // can't carry: writing it would leave anchor and rule disagreeing.
+        if (!prepared.canRealign && scope != RecurringWriteScope.ThisEvent) return
         pending = null
         _scopePrompt.value = null
-        viewModelScope.launch { write(prepared, scope) }
+        viewModelScope.launch {
+            write(prepared, scope)
+            busy = false
+        }
     }
 
     /** Dismiss the scope dialog without writing. */
     fun cancelScope() {
         pending = null
+        busy = false
         _scopePrompt.value = null
     }
 
     /** Put a completed move back where it came from. */
     fun undo(undo: MoveUndo) {
+        if (busy) return
+        busy = true
         _outcome.value = null
         viewModelScope.launch {
             _outcome.value = try {
@@ -194,6 +231,7 @@ class RescheduleViewModel @Inject constructor(
             } catch (e: Exception) {
                 MoveOutcome.Failed
             }
+            busy = false
         }
     }
 
@@ -219,23 +257,16 @@ class RescheduleViewModel @Inject constructor(
         val original = detail.toEditForm(request.beginMillis, request.endMillis, zone)
         val shifted = when (val target = request.target) {
             is MoveTarget.Start -> original.shiftedTo(target.instant, zone)
-            is MoveTarget.Day -> original.shiftedByDays(
+            is MoveTarget.ByDays -> original.shiftedByDays(target.days, zone)
-                (
-                    target.date.toEpochDays() -
-                        sourceDate(request.beginMillis, original.isAllDay, zone).toEpochDays()
-                    ).toInt(),
-            )
         }
         // A zero-distance drop is not a write. Matches the edit form's own
         // pristine-form no-op, and keeps a mis-aimed long press harmless.
         if (shifted == original) return null
 
-        val problems = shifted.problems()
+        // The UNTIL check is deferred to the write: how far the move reaches
-        if (EventFormProblem.RecurrenceEndsBeforeStart in problems) {
+        // decides which date has to clear it, and "only this event" writes an
-            _outcome.value = MoveOutcome.BlockedSeriesEnd
+        // exception row that no UNTIL constrains at all.
-            return null
+        if ((shifted.problems() - EventFormProblem.RecurrenceEndsBeforeStart).isNotEmpty()) {
-        }
-        if (problems.isNotEmpty()) {
             _outcome.value = MoveOutcome.Failed
             return null
         }
@@ -243,12 +274,23 @@ class RescheduleViewModel @Inject constructor(
         // An exception row stands alone whatever rule a sync adapter left on it.
         val isRecurring = original.rrule != null && !detail.isException
         val movedDay = shifted.start.date != original.start.date
+        // The series anchor moves by the same *wall-clock* shift as the dragged
+        // occurrence, so only a whole-day shift moves it by a predictable number
+        // of days — a drag that also changes the time of day would carry some
+        // anchor times of day across an extra midnight and leave the rule naming
+        // the wrong weekday. All-day forms shift as bare dates, so they always
+        // qualify.
+        val wholeDayShift = original.isAllDay || shifted.start.time == original.start.time
         val realigned = if (isRecurring && movedDay) {
+            if (wholeDayShift) {
                 realignRecurrence(
                     requireNotNull(original.rrule),
                     original.start.date,
                     shifted.start.date,
                 )
+            } else {
+                null
+            }
         } else {
             original.rrule
         }
@@ -260,11 +302,20 @@ class RescheduleViewModel @Inject constructor(
             updated = shifted.copy(rrule = realigned ?: original.rrule),
             isRecurring = isRecurring,
             canRealign = !isRecurring || !movedDay || realigned != null,
+            // Where the series row's own DTSTART lands, given the anchor moves by
+            // the same shift. Only meaningful under [wholeDayShift], which is the
+            // only case a wider-than-one-occurrence write is offered in.
+            newAnchorDate = anchorDate(detail, original, zone)
+                .plus(shifted.start.date.toEpochDays() - original.start.date.toEpochDays(), DateTimeUnit.DAY),
         )
     }
 
     private suspend fun write(prepared: PreparedMove, scope: RecurringWriteScope) {
         val request = prepared.request
+        if (endsBeforeItStarts(prepared, scope)) {
+            _outcome.value = MoveOutcome.BlockedSeriesEnd
+            return
+        }
         _outcome.value = try {
             when {
                 !prepared.isRecurring || scope == RecurringWriteScope.AllEvents ->
@@ -300,6 +351,39 @@ class RescheduleViewModel @Inject constructor(
         }
     }
 
+    /**
+     * Whether this write would leave a rule whose `UNTIL` precedes the first day
+     * it now applies to — the provider then generates nothing and the event
+     * silently disappears from every view.
+     *
+     * Which date has to clear `UNTIL` depends on how far the write reaches: a
+     * whole-series move carries the series *anchor*, a split starts a new series
+     * at the moved occurrence, and a single occurrence becomes an exception row
+     * that no `UNTIL` constrains. Testing the occurrence in every case would
+     * refuse the perfectly ordinary drag of a bounded series' last occurrence.
+     */
+    private fun endsBeforeItStarts(prepared: PreparedMove, scope: RecurringWriteScope): Boolean {
+        if (!prepared.isRecurring || scope == RecurringWriteScope.ThisEvent) return false
+        val rule = prepared.updated.rrule ?: return false
+        val end = parseSimpleRecurrence(rule)?.end as? RecurrenceEnd.Until ?: return false
+        val firstDay = if (scope == RecurringWriteScope.AllEvents) {
+            prepared.newAnchorDate
+        } else {
+            prepared.updated.start.date
+        }
+        return end.date < firstDay
+    }
+
+    /**
+     * The series row's own start date — for a recurring master, `EventDetail`
+     * carries the row's DTSTART rather than the tapped occurrence's. Read in the
+     * same anchoring the write path uses: UTC for an all-day series.
+     */
+    private fun anchorDate(detail: EventDetail, original: EventForm, zone: TimeZone): LocalDate =
+        detail.instance.start
+            .toLocalDateTime(if (original.isAllDay) TimeZone.UTC else original.resolvedZone(zone))
+            .date
+
     /**
      * Undo is offered only where the inverse is one symmetric write: a
      * non-recurring event (absolute DTSTART/DTEND) and a whole-series move (the
@@ -333,9 +417,4 @@ class RescheduleViewModel @Inject constructor(
         }
     }
 
-    /** The calendar day the dragged occurrence started on, as the grid shows it. */
-    private fun sourceDate(beginMillis: Long, isAllDay: Boolean, zone: TimeZone): LocalDate =
-        Instant.fromEpochMilliseconds(beginMillis)
-            .toLocalDateTime(if (isAllDay) TimeZone.UTC else zone)
-            .date
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/TimelineDrag.kt

@@ -7,7 +7,7 @@ import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.absoluteOffset
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.shape.RoundedCornerShape
@@ -26,6 +26,7 @@ import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.layout.LayoutCoordinates
 import androidx.compose.ui.layout.boundsInRoot
+import androidx.compose.ui.semantics.clearAndSetSemantics
 import androidx.compose.ui.layout.onGloballyPositioned
 import androidx.compose.ui.layout.positionInRoot
 import androidx.compose.ui.platform.LocalDensity
@@ -95,6 +96,13 @@ class TimelineGeometry {
     var edgePx: Float = 0f
     var stepPx: Float = 0f
     var days: List<LocalDate> = emptyList()
+
+    /**
+     * Whether the columns are laid out right-to-left. Pointer coordinates are
+     * never mirrored, but the grid is, so the leftmost column is the *last* day
+     * in Arabic — the mapping has to flip with it.
+     */
+    var isRtl: Boolean = false
 }
 
 /**
@@ -123,6 +131,14 @@ class TimelineDragController {
     private var grab = Offset.Zero
     private var pointer = Offset.Zero
 
+    /**
+     * The slot the block already occupied when it was picked up. A long press
+     * that never moves must write nothing — and because the target snaps to the
+     * grid, "nothing moved" is not the same as "the start is unchanged": an
+     * event at 09:07 resolves to 09:00 the instant it lifts.
+     */
+    private var originSlot: Pair<LocalDate, Int>? = null
+
     val isDragging: Boolean get() = liftedInstanceId != null
 
     fun begin(block: TimedBlock, pointerInRoot: Offset, blockInRoot: Offset) {
@@ -130,7 +146,9 @@ class TimelineDragController {
         liftedInstanceId = block.event.instanceId
         grab = pointerInRoot - blockInRoot
         pointer = pointerInRoot
+        originSlot = null
         recompute()
+        originSlot = drag?.slot
     }
 
     fun move(pointerInRoot: Offset) {
@@ -141,14 +159,20 @@ class TimelineDragController {
     fun cancel() {
         source = null
         liftedInstanceId = null
+        originSlot = null
         drag = null
     }
 
-    /** End the drag, handing back where it landed (null if it never resolved). */
+    /**
+     * End the drag, handing back where it landed — null when it never resolved,
+     * or when it landed back on the slot it started from.
+     */
     fun finish(): TimelineDrop? {
         val landed = drag
+        val origin = originSlot
         cancel()
-        return landed?.let { TimelineDrop(it.event, it.date, it.startMin) }
+        if (landed == null || landed.slot == origin) return null
+        return TimelineDrop(landed.event, landed.date, landed.startMin)
     }
 
     /**
@@ -171,11 +195,13 @@ class TimelineDragController {
         // fine and stays visible on the next day.
         val startMin = snapped.coerceIn(0, MINUTES_PER_DAY - DRAG_SNAP_MINUTES)
         val span = block.endMin - block.startMin
+        // The column the finger is over on screen, and the day that column shows.
         val column = ((pointer.x - origin.x) / columnPx).toInt().coerceIn(0, days.lastIndex)
+        val dayIndex = if (geometry.isRtl) days.lastIndex - column else column
         val height = maxOf(span / 60f * hourPx, MIN_EVENT_FRACTION * hourPx)
         drag = TimelineDrag(
             event = block.event,
-            date = days[column],
+            date = days[dayIndex],
             startMin = startMin,
             endMin = startMin + span,
             topLeftInRoot = Offset(
@@ -270,6 +296,9 @@ fun TimelineDragOverlay(controller: TimelineDragController, modifier: Modifier =
     Box(
         modifier = modifier
             .fillMaxSize()
+            // A copy of a block that is still in the tree behind it; announcing
+            // it again would just duplicate the event for the drag's duration.
+            .clearAndSetSemantics { }
             .onGloballyPositioned { origin = it.positionInRoot() },
     ) {
         val drag = controller.drag ?: return@Box
@@ -279,7 +308,9 @@ fun TimelineDragOverlay(controller: TimelineDragController, modifier: Modifier =
             formatMinuteOfDay(drag.endMin.coerceAtMost(MINUTES_PER_DAY), use24Hour, locale)
         Box(
             modifier = Modifier
-                .offset {
+                // Absolute: these are root coordinates, and the direction-aware
+                // offset would mirror them across the screen in an RTL layout.
+                .absoluteOffset {
                     IntOffset(
                         (drag.topLeftInRoot.x - origin.x).roundToInt(),
                         (drag.topLeftInRoot.y - origin.y).roundToInt(),

app/src/main/java/de/jeanlucmakiola/calendula/ui/day/DayScreen.kt

@@ -58,12 +58,14 @@ import androidx.compose.ui.input.nestedscroll.nestedScroll
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.onGloballyPositioned
 import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalLayoutDirection
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.semantics.contentDescription
 import androidx.compose.ui.semantics.customActions
 import androidx.compose.ui.semantics.semantics
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.LayoutDirection
 import androidx.compose.ui.unit.dp
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
@@ -536,6 +538,7 @@ private fun Timeline(
     val locale = currentLocale()
     val zoom = LocalTimelineZoom.current
     val density = LocalDensity.current
+    val isRtl = LocalLayoutDirection.current == LayoutDirection.Rtl
 
     // BoxWithConstraints rather than Box: the fit-the-whole-day scale needs the
     // timeline's own viewport height, which is only known here — below the top
@@ -612,6 +615,7 @@ private fun Timeline(
                                 it.columnGapPx = 0f
                                 it.columnWidthPx = coords.size.width.toFloat()
                                 it.days = listOf(state.date)
+                                it.isRtl = isRtl
                             }
                         },
                 )
@@ -727,15 +731,16 @@ private fun EventBlock(
     val moveAction = eventMoveAction(block.event)
     // A block clipped at the top continues from the previous day: its top edge is
     // midnight, not the event's start, so dragging it would invent a time.
+    val draggable = moveAction != null && block.beginsOn(date, zone)
     val dragModifier = rememberEventDragSource(
-        enabled = moveAction != null && block.beginsOn(date, zone),
+        enabled = draggable,
         key = block.event.instanceId,
         onPickUp = { pointer, blockRoot, _ -> dragController.begin(block, pointer, blockRoot) },
         onMove = dragController::move,
         onDrop = { dragController.finish()?.let(onDrop) },
         onCancel = dragController::cancel,
     )
-    val lifted = dragController.liftedInstanceId == block.event.instanceId
+    val lifted = draggable && dragController.liftedInstanceId == block.event.instanceId
     Box(
         modifier = modifier
             // The source stays put as a ghost while its floating copy travels.

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthDrag.kt

@@ -20,6 +20,12 @@ class MonthRowGeometry(
     /** The row's day-column box — the space chip offsets are measured in. */
     val cell: LayoutCoordinates,
     val columnWidthPx: Float,
+    /**
+     * Whether the columns are laid out right-to-left. Pointer coordinates are
+     * never mirrored, but the grid is, so the leftmost column is the *last* day
+     * in Arabic.
+     */
+    val isRtl: Boolean,
 )
 
 /** A chip in flight, in root coordinates so it can be drawn in an overlay. */
@@ -127,7 +133,7 @@ class MonthDragController {
                 val column = ((pointer.x - bounds.left) / row.columnWidthPx)
                     .toInt()
                     .coerceIn(0, row.days.lastIndex)
-                row.days[column]
+                row.days[if (row.isRtl) row.days.lastIndex - column else column]
             }
         }
         drag = MonthChipDrag(

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthScreen.kt

@@ -78,6 +78,10 @@ import androidx.compose.ui.draw.clip
 import androidx.compose.ui.draw.clipToBounds
 import androidx.compose.ui.draw.drawBehind
 import androidx.compose.ui.geometry.CornerRadius
+import androidx.compose.foundation.layout.absoluteOffset
+import androidx.compose.ui.platform.LocalLayoutDirection
+import androidx.compose.ui.semantics.clearAndSetSemantics
+import androidx.compose.ui.unit.LayoutDirection
 import androidx.compose.ui.geometry.Offset
 import kotlin.math.roundToInt
 import de.jeanlucmakiola.calendula.ui.common.rememberDragSurface
@@ -472,6 +476,9 @@ private fun MonthDragOverlay(controller: MonthDragController) {
     Box(
         modifier = Modifier
             .fillMaxSize()
+            // A copy of a chip that is still in the tree behind it; announcing it
+            // again would just duplicate the event for the drag's duration.
+            .clearAndSetSemantics { }
             .onGloballyPositioned { origin = it.positionInRoot() },
     ) {
         val drag = controller.drag ?: return@Box
@@ -481,7 +488,9 @@ private fun MonthDragOverlay(controller: MonthDragController) {
             continuesLeft = false,
             continuesRight = false,
             modifier = Modifier
-                .offset {
+                // Absolute: these are root coordinates, and the direction-aware
+                // offset would mirror them across the screen in an RTL layout.
+                .absoluteOffset {
                     IntOffset(
                         (drag.topLeftInRoot.x - origin.x).roundToInt(),
                         (drag.topLeftInRoot.y - origin.y).roundToInt(),
@@ -1802,7 +1811,7 @@ private fun MonthWeekRow(
     val bandCoordinates = remember { arrayOfNulls<LayoutCoordinates>(1) }
     val density = LocalDensity.current
     val rowHeightPx = with(density) { EVENT_ROW_HEIGHT.toPx() }
-    val zone = remember { TimeZone.currentSystemDefault() }
+    val isRtl = LocalLayoutDirection.current == LayoutDirection.Rtl
     val dragging = dragController?.isDragging == true
     DisposableEffect(rowToken, dragController) {
         onDispose { dragController?.removeRow(rowToken) }
@@ -1831,6 +1840,7 @@ private fun MonthWeekRow(
                             days = week.days,
                             cell = coords,
                             columnWidthPx = coords.size.width / 7f,
+                            isRtl = isRtl,
                         ),
                     )
                 }
@@ -1841,7 +1851,7 @@ private fun MonthWeekRow(
                         controller = dragController,
                         band = bandCoordinates,
                         rowHeightPx = rowHeightPx,
-                        zone = zone,
+                        isRtl = isRtl,
                     ),
                 ),
         ) {
@@ -2083,7 +2093,7 @@ private fun monthChipDragModifier(
     controller: MonthDragController?,
     band: Array<LayoutCoordinates?>,
     rowHeightPx: Float,
-    zone: TimeZone,
+    isRtl: Boolean,
 ): Modifier = rememberDragSurface(
     enabled = moveScope != null && controller != null,
     key = week.days.first(),
@@ -2092,18 +2102,20 @@ private fun monthChipDragModifier(
         val columnPx = size.width / 7f
         val bandY = if (bandTop == null) -1f else local.y - (bandTop - nodeInRoot.y)
         val lane = (bandY / rowHeightPx).toInt()
+        // The column under the finger on screen, and the day that column shows.
         val column = (local.x / columnPx).toInt()
+        val dayIndex = if (isRtl) week.days.lastIndex - column else column
         val event = if (bandY < 0f || columnPx <= 0f) {
             null
         } else {
-            week.chipAt(column, lane, MAX_EVENT_ROWS)
+            week.chipAt(dayIndex, lane, MAX_EVENT_ROWS)
         }
         if (event == null || moveScope?.allows(event) != true) {
             false
         } else {
             controller?.begin(
                 event = event,
-                grabDate = week.days[column],
+                grabDate = week.days[dayIndex],
                 pointerInRoot = pointerInRoot,
                 chipInRoot = Offset(
                     x = nodeInRoot.x + column * columnPx,
@@ -2117,19 +2129,17 @@ private fun monthChipDragModifier(
     onMove = { controller?.move(it) },
     onDrop = {
         controller?.finish()?.let { drop ->
-            val delta = drop.targetDate.toEpochDays() - drop.grabDate.toEpochDays()
+            // How many columns the finger crossed — grabbing the middle of a
-            if (delta != 0L) {
+            // multi-day bar shifts the event by what the finger travelled, not
+            // to where it landed.
+            val delta = (drop.targetDate.toEpochDays() - drop.grabDate.toEpochDays()).toInt()
+            if (delta != 0) {
                 moveScope?.move(
                     MoveRequest(
                         eventId = drop.event.eventId,
                         beginMillis = drop.event.start.toEpochMilliseconds(),
                         endMillis = drop.event.end.toEpochMilliseconds(),
-                        // The event's own first day plus the columns crossed —
+                        target = MoveTarget.ByDays(delta),
-                        // grabbing the middle of a multi-day bar shifts it by
-                        // what the finger travelled, not to where it landed.
-                        target = MoveTarget.Day(
-                            drop.event.spanFirstDay(zone).plus(delta.toInt(), DateTimeUnit.DAY),
-                        ),
                     ),
                 )
             }

app/src/main/java/de/jeanlucmakiola/calendula/ui/week/WeekScreen.kt

@@ -64,6 +64,7 @@ import androidx.compose.ui.input.nestedscroll.nestedScroll
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.layout.onGloballyPositioned
 import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.platform.LocalLayoutDirection
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.semantics.contentDescription
 import androidx.compose.ui.semantics.customActions
@@ -71,6 +72,7 @@ import androidx.compose.ui.semantics.semantics
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.LayoutDirection
 import androidx.compose.ui.unit.dp
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
@@ -672,6 +674,7 @@ private fun Timeline(
     val locale = currentLocale()
     val zoom = LocalTimelineZoom.current
     val density = LocalDensity.current
+    val isRtl = LocalLayoutDirection.current == LayoutDirection.Rtl
 
     // BoxWithConstraints rather than Box: the fit-the-whole-day scale needs the
     // timeline's own viewport height, which is only known here — below the top
@@ -742,6 +745,7 @@ private fun Timeline(
                                 it.columnGapPx = gap
                                 it.columnWidthPx = (coords.size.width + gap) / state.days.size
                                 it.days = state.days
+                                it.isRtl = isRtl
                             }
                         },
                     horizontalArrangement = Arrangement.spacedBy(COLUMN_GAP),
@@ -895,15 +899,16 @@ private fun EventBlock(
     val moveAction = eventMoveAction(block.event)
     // A block clipped at the top continues from the previous day: its top edge is
     // midnight, not the event's start, so dragging it would invent a time.
+    val draggable = moveAction != null && block.beginsOn(date, zone)
     val dragModifier = rememberEventDragSource(
-        enabled = moveAction != null && block.beginsOn(date, zone),
+        enabled = draggable,
         key = block.event.instanceId,
         onPickUp = { pointer, blockRoot, _ -> dragController.begin(block, pointer, blockRoot) },
         onMove = dragController::move,
         onDrop = { dragController.finish()?.let(onDrop) },
         onCancel = dragController::cancel,
     )
-    val lifted = dragController.liftedInstanceId == block.event.instanceId
+    val lifted = draggable && dragController.liftedInstanceId == block.event.instanceId
     Box(
         modifier = (if (dimmed) modifier.alpha(EventDimAlpha) else modifier)
             // The source stays put as a ghost while its floating copy travels.

app/src/test/java/de/jeanlucmakiola/calendula/domain/EventShiftTest.kt

@@ -109,7 +109,7 @@ class EventShiftTest {
 
         assertThat(allDay.shiftedTo(target, berlin)).isEqualTo(allDay)
 
-        val moved = allDay.shiftedByDays(3)
+        val moved = allDay.shiftedByDays(3, berlin)
         assertThat(moved.start.date).isEqualTo(LocalDate(2026, 6, 14))
         assertThat(moved.end.date).isEqualTo(LocalDate(2026, 6, 14))
         // The placeholder times exist only for a switch back to timed; untouched.
@@ -123,15 +123,34 @@ class EventShiftTest {
             end = LocalDateTime(LocalDate(2026, 6, 13), LocalTime(2, 0)),
         )
 
-        val moved = multiDay.shiftedByDays(-2)
+        val moved = multiDay.shiftedByDays(-2, berlin)
 
         assertThat(moved.start).isEqualTo(LocalDateTime(LocalDate(2026, 6, 9), LocalTime(22, 0)))
         assertThat(moved.end).isEqualTo(LocalDateTime(LocalDate(2026, 6, 11), LocalTime(2, 0)))
     }
 
+    @Test
+    fun `a timed shift onto a DST changeover keeps the real length, not the wall clock`() {
+        // 22:00 Sat -> 04:00 Sun is six real hours; the target Sunday is the one
+        // Berlin springs forward on. Keeping wall clock would write a five-hour
+        // DURATION for the whole series.
+        val overnight = form(
+            start = LocalDateTime(LocalDate(2026, 3, 21), LocalTime(22, 0)),
+            end = LocalDateTime(LocalDate(2026, 3, 22), LocalTime(4, 0)),
+        )
+
+        val moved = overnight.shiftedByDays(7, berlin)
+
+        assertThat(moved.start).isEqualTo(LocalDateTime(LocalDate(2026, 3, 28), LocalTime(22, 0)))
+        assertThat(moved.end.toInstant(berlin) - moved.start.toInstant(berlin))
+            .isEqualTo(overnight.end.toInstant(berlin) - overnight.start.toInstant(berlin))
+        // The wall-clock end therefore lands an hour later than a naive +7 days.
+        assertThat(moved.end).isEqualTo(LocalDateTime(LocalDate(2026, 3, 29), LocalTime(5, 0)))
+    }
+
     @Test
     fun `shifting by no days is a no-op`() {
         val original = form()
-        assertThat(original.shiftedByDays(0)).isEqualTo(original)
+        assertThat(original.shiftedByDays(0, berlin)).isEqualTo(original)
     }
 }

app/src/test/java/de/jeanlucmakiola/calendula/domain/RecurrenceRealignTest.kt

@@ -23,21 +23,28 @@ class RecurrenceRealignTest {
     }
 
     @Test
-    fun `a monthly BYMONTHDAY rule follows the day of the month`() {
+    fun `a day-of-month rule is refused, because the anchor moves by days not dates`() {
+        // BYMONTHDAY=28 with a January anchor, occurrence Feb 28 dragged to Mar 1:
+        // the rebuilt rule would say the 1st while the anchor became Jan 29 — a
+        // DTSTART that is not an instance of its own rule. Weekday arithmetic is
+        // uniform mod 7 and survives the same shift; day-of-month is not.
+        assertThat(realignRecurrence("FREQ=MONTHLY;BYMONTHDAY=8", monday, wednesday)).isNull()
         assertThat(
-            realignRecurrence("FREQ=MONTHLY;BYMONTHDAY=8", monday, wednesday),
+            realignRecurrence("FREQ=YEARLY;BYMONTH=6;BYMONTHDAY=8", monday, LocalDate(2026, 7, 20)),
-        ).isEqualTo("FREQ=MONTHLY;BYMONTHDAY=10")
+        ).isNull()
     }
 
     @Test
-    fun `a yearly rule realigns both month and day`() {
+    fun `BYDAY on a non-weekly rule is refused`() {
-        assertThat(
+        // "every Monday of the month" is a shape this has not been reasoned about,
-            realignRecurrence(
+        // and parseSimpleRecurrence can't read it either — so the UNTIL guard
-                "FREQ=YEARLY;BYMONTH=6;BYMONTHDAY=8",
+        // downstream would be blind to it.
-                monday,
+        assertThat(realignRecurrence("FREQ=MONTHLY;BYDAY=MO", monday, wednesday)).isNull()
-                LocalDate(2026, 7, 20),
+    }
-            ),
+
-        ).isEqualTo("FREQ=YEARLY;BYMONTH=7;BYMONTHDAY=20")
+    @Test
+    fun `a rule with no FREQ is refused`() {
+        assertThat(realignRecurrence("INTERVAL=2;BYDAY=MO", monday, wednesday)).isNull()
     }
 
     @Test
@@ -71,11 +78,28 @@ class RecurrenceRealignTest {
 
     @Test
     fun `parts we cannot reason about are refused rather than guessed`() {
-        assertThat(realignRecurrence("FREQ=MONTHLY;BYDAY=MO;BYSETPOS=1", monday, wednesday))
+        assertThat(realignRecurrence("FREQ=WEEKLY;BYDAY=MO;BYSETPOS=1", monday, wednesday))
             .isNull()
         assertThat(realignRecurrence("FREQ=YEARLY;BYYEARDAY=159", monday, wednesday)).isNull()
     }
 
+    @Test
+    fun `everything realignable is also a rule the UNTIL guard can read`() {
+        // problems() checks UNTIL through parseSimpleRecurrence; a rule this
+        // realigns but that parser rejects would move a series past its own end
+        // unchecked. Accepting only weekly BYDAY keeps the two in step.
+        val realignable = listOf(
+            "FREQ=WEEKLY;BYDAY=MO",
+            "FREQ=WEEKLY;INTERVAL=2;BYDAY=MO;UNTIL=20261231T225959Z",
+            "FREQ=DAILY;COUNT=5",
+        )
+        realignable.forEach { rule ->
+            assertThat(realignRecurrence(rule, monday, wednesday)).isNotNull()
+            assertThat(parseSimpleRecurrence(realignRecurrence(rule, monday, wednesday)!!))
+                .isNotNull()
+        }
+    }
+
     @Test
     fun `a leading RRULE prefix is preserved`() {
         assertThat(realignRecurrence("RRULE:FREQ=WEEKLY;BYDAY=MO", monday, wednesday))

app/src/test/java/de/jeanlucmakiola/calendula/ui/common/RescheduleViewModelTest.kt

@@ -45,12 +45,14 @@ class RescheduleViewModelTest {
     @BeforeEach fun setUp() = Dispatchers.setMain(dispatcher)
     @AfterEach fun tearDown() = Dispatchers.resetMain()
 
-    // Monday 2026-06-08, midday UTC — midday so the device zone can't move the
+    // Monday 2026-06-08, midday in whatever zone the test JVM runs in — resolved
-    // weekday out from under the BYDAY assertions.
+    // in the device zone rather than UTC, because that is the zone the drop path
+    // reads dates back in, and at UTC+13 a UTC midday is already Tuesday. The
+    // BYDAY assertions below depend on this really being a Monday.
     private val monday = LocalDate(2026, 6, 8)
-    private val wednesday = LocalDate(2026, 6, 10)
+    private val beginMillis = LocalDateTime(monday, LocalTime(12, 0))
-    private val beginMillis =
+        .toInstant(TimeZone.currentSystemDefault())
-        LocalDateTime(monday, LocalTime(12, 0)).toInstant(TimeZone.UTC).toEpochMilliseconds()
+        .toEpochMilliseconds()
     private val endMillis = beginMillis + 3_600_000L
 
     private fun cal(
@@ -103,7 +105,7 @@ class RescheduleViewModelTest {
         eventId = 42L,
         beginMillis = beginMillis,
         endMillis = endMillis,
-        target = MoveTarget.Day(wednesday),
+        target = MoveTarget.ByDays(2),
     )
 
     /** A drop an hour later, expressed the way a timeline drag expresses one. */
@@ -262,13 +264,62 @@ class RescheduleViewModelTest {
         val vm = viewModel(tempDir, fake)
 
         vm.move(toWednesday())
+        vm.moveWithScope(RecurringWriteScope.AllEvents)
         advanceUntilIdle()
 
         assertThat(vm.outcome.value).isEqualTo(MoveOutcome.BlockedSeriesEnd)
-        assertThat(vm.scopePrompt.value).isNull()
         assertThat(fake.updatedEvents).isEmpty()
     }
 
+    @Test
+    fun `dragging a bounded series' last occurrence still allows moving just it`(
+        @TempDir tempDir: Path,
+    ) = runTest(dispatcher) {
+        val fake = FakeCalendarDataSource().apply {
+            eventDetailResult = { detail(rrule = "FREQ=WEEKLY;UNTIL=20260608T120000Z") }
+        }
+        val vm = viewModel(tempDir, fake)
+
+        vm.move(toWednesday())
+        advanceUntilIdle()
+        // The block belongs to the write, not to the drop: an exception row is
+        // constrained by no UNTIL, so this scope must still be on offer.
+        assertThat(vm.scopePrompt.value).isNotNull()
+
+        vm.moveWithScope(RecurringWriteScope.ThisEvent)
+        advanceUntilIdle()
+
+        assertThat(fake.updatedOccurrences).hasSize(1)
+        assertThat(vm.outcome.value).isInstanceOf(MoveOutcome.Moved::class.java)
+    }
+
+    @Test
+    fun `a drag that changes both the day and the time cannot move the series`(
+        @TempDir tempDir: Path,
+    ) = runTest(dispatcher) {
+        val fake = FakeCalendarDataSource().apply {
+            eventDetailResult = { detail(rrule = "FREQ=WEEKLY;BYDAY=MO") }
+        }
+        val vm = viewModel(tempDir, fake)
+
+        // Onto the next day *and* two hours later: the anchor moves by the same
+        // wall-clock shift, so an anchor late enough in the day would cross two
+        // midnights and land on a weekday the rebuilt rule doesn't name.
+        vm.move(
+            MoveRequest(
+                eventId = 42L,
+                beginMillis = beginMillis,
+                endMillis = endMillis,
+                target = MoveTarget.Start(
+                    Instant.fromEpochMilliseconds(beginMillis + 26 * 3_600_000L),
+                ),
+            ),
+        )
+        advanceUntilIdle()
+
+        assertThat(vm.scopePrompt.value).isEqualTo(MoveScopePrompt(occurrenceOnly = true))
+    }
+
     @Test
     fun `a zero-distance drop writes nothing and says nothing`(
         @TempDir tempDir: Path,

docs/ARCHITECTURE.md

@@ -123,21 +123,45 @@ Two things the drag has to get right that the edit screen sidesteps:
 - **`RRULE` day parts go stale.** `buildEventUpdateValues` writes the rule
   verbatim while `DTSTART` moves, so dragging a `FREQ=WEEKLY;BYDAY=MO` occurrence
   onto a Wednesday under *All events* leaves a Wednesday anchor under a Monday
-  rule and the series does not move. `realignRecurrence` re-derives `BYDAY` /
+  rule and the series does not move. `realignRecurrence` re-derives `BYDAY`, and
-  `BYMONTHDAY` / `BYMONTH` from the new date, and returns **null** for rules one
+  returns **null** for everything else — the drop then offers only *this event*,
-  moved occurrence cannot resolve (`BYDAY=MO,WE`, `2TH`, `BYSETPOS`) — the drop
+  whose exception row carries no rule at all. The same staleness is reachable
-  then offers only *this event*, whose exception row carries no rule at all. The
+  from the edit screen; wiring it there too is a separate change.
-  same staleness is reachable from the edit screen; wiring it there too is a
+
-  separate change.
+  What the rule has to agree with is the **series anchor**, not the occurrence
+  being dragged, and the anchor moves by the same *wall-clock* shift. Only two
+  things survive that intact, which is exactly the envelope the realigner accepts:
+  weekday (uniform mod 7, so every anchor time of day crosses the same number of
+  midnights) and a shift that is a whole number of days (otherwise a late-enough
+  anchor crosses one midnight more than the occurrence did). Day-of-month is not
+  uniform — `BYMONTHDAY=28` with a January anchor, occurrence Feb 28 dragged to
+  Mar 1, would leave a Jan 29 anchor under a `BYMONTHDAY=1` rule, a DTSTART that
+  is not an instance of its own rule and a phantom occurrence on any client that
+  trusts it. Those drags may only move the one occurrence.
+
+  The accepted parts are deliberately a **subset** of what `parseSimpleRecurrence`
+  understands, so anything realignable is also a rule whose `UNTIL` the guard
+  below can actually read.
 - **The eligibility gate is load-bearing.** Nothing below the UI refuses a
   write, so `CalendarSource.allowsEventMove` is what keeps read-only and
   contact-managed events from being dragged. It is deliberately *not*
   `isEventTarget`: a managed event is editable (reminders, notes) yet must never
   move, while a switched-off calendar renders nothing to grab anyway.
 
-`EventForm.problems()` runs before the write, so a drop that would push a series
+A drop that would push a series past its own `UNTIL` — the provider then
-past its own `UNTIL` — which makes the provider generate zero occurrences and the
+generates zero occurrences and the event vanishes from every view — is refused
-event vanish — is refused rather than written.
+rather than written. The check happens at **write** time, not at drop time,
+because which date has to clear `UNTIL` depends on how far the write reaches: a
+whole-series move carries the *anchor*, a split starts a new series at the moved
+occurrence, and a single occurrence becomes an exception row that no `UNTIL`
+constrains. Testing the occurrence in every case would refuse the perfectly
+ordinary drag of a bounded series' last occurrence.
+
+Two known limitations of a whole-series move, both shared with the edit screen's
+own *All events* time save rather than introduced here — a drag just makes them
+one gesture away: the series' `EXDATE` stamps and its exception rows are **not**
+re-anchored, so previously deleted occurrences can reappear and previously
+modified ones stay behind while the rest of the series moves.
 
 ### Event time zones
 
@@ -186,7 +210,10 @@ shape (`DURATION` normalises to `P<n>S`/`P<n>D`, `EVENT_TIMEZONE` is stamped
 concrete), and it is offered only where the inverse is one symmetric write — a
 one-off event or a whole-series shift. *This event* leaves an exception row
 behind and *this and following* splits the series; neither is undone by shifting
-back, so both get a plain confirmation.
+back, so both get a plain confirmation. Two further gaps, both narrow and
+accepted: a shift whose *anchor* crosses a DST gap is not invertible in wall
+clock (the −Δ normalises back to where it started), and an undo after a
+concurrent remote time change overwrites it, exactly as the forward move would.
 
 ## Reminder delivery