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

@@ -101,6 +101,7 @@ import de.jeanlucmakiola.calendula.ui.common.rememberCalendarSlideSpec
 import de.jeanlucmakiola.calendula.ui.common.next
 import de.jeanlucmakiola.floret.time.isoWeekNumber
 import kotlinx.coroutines.flow.distinctUntilChanged
+import kotlinx.coroutines.flow.filterNotNull
 import kotlinx.coroutines.launch
 import kotlinx.datetime.DayOfWeek
 import kotlinx.datetime.LocalDate
@@ -176,10 +177,18 @@ fun MonthScreen(
     LaunchedEffect(listState, continuous) {
         if (!continuous) return@LaunchedEffect
         snapshotFlow {
+            // Empty before the first layout pass — and reporting a default of 0
+            // for it would claim January 1900 is on screen and drag the whole
+            // window back there.
             val visible = listState.layoutInfo.visibleItemsInfo
-            monthIndexForItem(visible.firstOrNull()?.index ?: 0) to
+            if (visible.isEmpty()) {
-                monthIndexForItem(visible.lastOrNull()?.index ?: 0)
+                null
+            } else {
+                monthIndexForItem(visible.first().index) to
+                    monthIndexForItem(visible.last().index)
             }
+        }
+            .filterNotNull()
             .distinctUntilChanged()
             .collect { (first, last) -> viewModel.onVisibleMonthsChanged(first, last) }
     }

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

@@ -148,7 +148,8 @@ class MonthViewModel @Inject constructor(
      * close enough to a loaded edge to warrant a wider query.
      */
     fun onVisibleMonthsChanged(firstIndex: Int, lastIndex: Int) {
-        nextLoadWindow(_loadedMonths.value, firstIndex, lastIndex)?.let { _loadedMonths.value = it }
+        nextLoadWindow(_loadedMonths.value, firstIndex, lastIndex)
+            ?.let { _loadedMonths.value = clampMonthWindow(it) }
     }
 
     private fun buildContinuousState(
@@ -402,6 +403,16 @@ internal fun itemIndexForMonth(monthIndex: Int): Int = monthIndex * 2
 
 internal fun monthIndexForItem(itemIndex: Int): Int = itemIndex / 2
 
+/**
+ * Hold a load window inside the months that actually exist. The pad
+ * [nextLoadWindow] adds can otherwise push an index past either end of the
+ * 1900–2100 span, and [yearMonthForIndex] has no month to hand back for one.
+ */
+internal fun clampMonthWindow(window: IntRange): IntRange {
+    val last = continuousMonthCount() - 1
+    return window.first.coerceIn(0, last)..window.last.coerceIn(0, last)
+}
+
 internal fun firstOfMonth(ym: YearMonth): LocalDate = LocalDate(ym.year, ym.month, 1)
 
 /** How many week rows [ym] needs under [weekStart] — 4 (a non-leap February), 5 or 6. */

app/src/test/java/de/jeanlucmakiola/calendula/ui/month/ContinuousMonthIndexTest.kt

@@ -88,6 +88,25 @@ class ContinuousMonthIndexTest {
         }
     }
 
+    @Test
+    fun `a window is clamped to the months that exist`() {
+        // The regression this guards: an empty visible-items list reported as
+        // "month 0", whose padded window ran to -4 — and yearMonthForIndex has
+        // no month for that, so the whole grid failed with "couldn't read".
+        assertThat(clampMonthWindow(-4..4)).isEqualTo(0..4)
+        val last = continuousMonthCount() - 1
+        assertThat(clampMonthWindow((last - 2)..(last + 4))).isEqualTo((last - 2)..last)
+        assertThat(clampMonthWindow(10..20)).isEqualTo(10..20)
+    }
+
+    @Test
+    fun `every index a clamped window can hold resolves to a month`() {
+        listOf(clampMonthWindow(-4..4), clampMonthWindow((continuousMonthCount() - 1)..(continuousMonthCount() + 3)))
+            .forEach { window ->
+                window.forEach { index -> yearMonthForIndex(index) }
+            }
+    }
+
     @Test
     fun `firstOfMonth is the 1st`() {
         assertThat(firstOfMonth(jun26)).isEqualTo(LocalDate(2026, 6, 1))