Refactor collapse animation to overlay selected row instead of shrinking rows

Selected row now moves up via offset + zIndex to cover other rows, which
stay in place. Collapse offset calculation excludes MonthHeader and
WeekdayHeader heights so only the grid portion is collapsed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarMonthPage.kt

@@ -3,7 +3,7 @@ package plus.rua.project.ui
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
-import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
 import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
@@ -11,9 +11,11 @@ import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clipToBounds
 import androidx.compose.ui.layout.onSizeChanged
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.zIndex
 import kotlinx.datetime.DatePeriod
 import kotlinx.datetime.LocalDate
 import kotlinx.datetime.minus
@@ -22,12 +24,13 @@ import kotlinx.datetime.plus
 /**
  * 月度日历网格页，6×7 布局，支持折叠动画。
  *
- * 折叠时选中行保持原高，上方行向上收缩、下方行向下收缩，模拟"挤压"效果。
+ * 折叠时选中行保持原高并向上移动覆盖其他行，其他行保持原位不动。
+ * 选中行通过 offset + zIndex 实现覆盖效果。
  *
  * @param year 年份
  * @param month 月份（1-12）
  * @param selectedDate 当前选中日期
- * @param today 今天的日期，用于高亮标记
+ * @param today 今天的日期
  * @param onDateClick 日期点击回调
  * @param collapseProgress 折叠进度，0f=展开（6行），1f=折叠（仅选中行可见）
  * @param modifier 外部布局修饰符
@@ -53,54 +56,46 @@ fun CalendarMonthPage(
     }
 
     var rowHeightPx by remember { mutableIntStateOf(0) }
-    val rowMeasured = rowHeightPx > 0
 
-    Column(modifier = modifier) {
+    // 选中行上移距离 = 上方行数 × 行高 × progress
+    val selectedOffsetPx = if (rowHeightPx > 0) {
+        -(selectedWeekIndex.toFloat() * rowHeightPx.toFloat() * collapseProgress)
+    } else {
+        0f
+    }
+    val selectedOffsetDp = with(density) { selectedOffsetPx.toDp() }
+
+    Column(modifier = modifier.clipToBounds()) {
         weeks.forEachIndexed { weekIndex, week ->
-            val isAboveSelected = weekIndex < selectedWeekIndex
+            val isSelected = weekIndex == selectedWeekIndex
-            val isBelowSelected = weekIndex > selectedWeekIndex
 
-            val rowScale = when {
+            Row(
-                isAboveSelected || isBelowSelected -> 1f - collapseProgress
+                modifier = Modifier
-                else -> 1f
+                    .fillMaxWidth()
-            }
+                    .zIndex(if (isSelected) 1f else 0f)
-
+                    .then(
-            val rowHeightDp = if (rowMeasured && rowScale > 0.01f) {
+                        if (isSelected && rowHeightPx > 0) {
-                with(density) { (rowHeightPx * rowScale).toDp() }
+                            Modifier.offset(y = selectedOffsetDp)
-            } else if (!rowMeasured) {
+                        } else {
-                // First frame: let aspectRatio determine height naturally
+                            Modifier
-                null
+                        }
-            } else {
+                    )
-                0.dp
+                    .onSizeChanged { size ->
-            }
+                        if (size.height > 0 && rowHeightPx == 0) {
-
+                            rowHeightPx = size.height
-            val shouldShow = rowHeightDp == null || rowHeightDp > 0.dp
-
-            if (shouldShow) {
-                Row(
-                    modifier = Modifier
-                        .fillMaxWidth()
-                        .then(
-                            if (rowHeightDp != null) Modifier.height(rowHeightDp)
-                            else Modifier
-                        )
-                        .onSizeChanged { size ->
-                            if (size.height > 0 && !rowMeasured) {
-                                rowHeightPx = size.height
-                            }
                         }
-                        .padding(vertical = 2.dp)
-                ) {
-                    week.forEach { dayData ->
-                        DayCell(
-                            date = dayData.date,
-                            isCurrentMonth = dayData.isCurrentMonth,
-                            isSelected = dayData.date == selectedDate,
-                            isToday = dayData.date == today,
-                            onClick = { onDateClick(dayData.date) },
-                            modifier = Modifier.weight(1f)
-                        )
                     }
+                    .padding(vertical = 2.dp)
+            ) {
+                week.forEach { dayData ->
+                    DayCell(
+                        date = dayData.date,
+                        isCurrentMonth = dayData.isCurrentMonth,
+                        isSelected = dayData.date == selectedDate,
+                        isToday = dayData.date == today,
+                        onClick = { onDateClick(dayData.date) },
+                        modifier = Modifier.weight(1f)
+                    )
                 }
             }
         }
@@ -112,13 +107,12 @@ private data class DayData(
     val isCurrentMonth: Boolean
 )
 
-@Suppress("DEPRECATION") // monthNumber 无替代 API，kotlinx-datetime 尚未提供新接口
+@Suppress("DEPRECATION")
 private fun generateMonthDays(year: Int, month: Int): List<DayData> {
     val firstOfMonth = LocalDate(year, month, 1)
     val offset = firstOfMonth.dayOfWeek.ordinal
     val startDate = firstOfMonth.minus(DatePeriod(days = offset))
 
-    // 6行×7列=42格，覆盖跨月首尾周，保证网格完整
     return (0 until 42).map { i ->
         val date = startDate.plus(DatePeriod(days = i))
         DayData(

shared/src/commonMain/kotlin/plus/rua/project/ui/CalendarMonthView.kt

@@ -46,13 +46,16 @@ fun CalendarMonthView(
     var calendarHeightPx by remember { mutableIntStateOf(0) }
     var screenHeightPx by remember { mutableIntStateOf(0) }
     var expandedCalendarHeightPx by remember { mutableIntStateOf(0) }
+    var monthHeaderHeightPx by remember { mutableIntStateOf(0) }
+    var weekdayHeaderHeightPx by remember { mutableIntStateOf(0) }
 
-    // collapseProgress: 0f=月视图(6行), 1f=周视图(1行)
+    // 日历网格高度 = 总高度 - MonthHeader - WeekdayHeader
-    // 折叠偏移量 = 进度 × 展开高度的5/6（保留1行可见）
+    val expandedGridHeightPx = expandedCalendarHeightPx - monthHeaderHeightPx - weekdayHeaderHeightPx
+    // 折叠偏移量 = 进度 × 网格5行高度（保留1行可见）
     val collapseOffsetPx = if (viewModel.isCollapsed) {
         0
     } else {
-        -(viewModel.collapseProgress * expandedCalendarHeightPx * 5f / 6f).toInt()
+        -(viewModel.collapseProgress * expandedGridHeightPx * 5f / 6f).toInt()
     }
     val cardTopPx = if (viewModel.isCollapsed) {
         calendarHeightPx
@@ -79,9 +82,16 @@ fun CalendarMonthView(
             MonthHeader(
                 year = currentYear,
                 month = currentMonth,
-                weekNumber = viewModel.getIsoWeekNumber(viewModel.selectedDate)
+                weekNumber = viewModel.getIsoWeekNumber(viewModel.selectedDate),
+                modifier = Modifier.onSizeChanged { size ->
+                    monthHeaderHeightPx = size.height
+                }
+            )
+            WeekdayHeader(
+                modifier = Modifier.fillMaxWidth().onSizeChanged { size ->
+                    weekdayHeaderHeightPx = size.height
+                }
             )
-            WeekdayHeader(modifier = Modifier.fillMaxWidth())
             if (viewModel.isCollapsed) {
                 WeekPager(
                     selectedDate = viewModel.selectedDate,