[SplitPane] Add ability to hide first pane

This commit adds the ability to hide the first pane in a SplitPane.

- Adds `firstVisible
` property to `SplitPaneState` to control the visibility of the first pane.
- Updates `SplitPane` layout to handle the visibility of the first pane.
- Adds buttons to the demo to toggle the visibility of the first pane.

components/SplitPane/demo/src/jvmMain/kotlin/org/jetbrains/compose/splitpane/demo/Main.kt

@@ -23,7 +23,9 @@ import androidx.compose.ui.graphics.SolidColor
 import androidx.compose.ui.input.pointer.PointerIcon
 import androidx.compose.ui.input.pointer.pointerHoverIcon
 import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.DpSize
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.window.WindowState
 import androidx.compose.ui.window.singleWindowApplication
 import org.jetbrains.compose.splitpane.ExperimentalSplitPaneApi
 import org.jetbrains.compose.splitpane.HorizontalSplitPane
@@ -36,8 +38,12 @@ private fun Modifier.cursorForHorizontalResize(): Modifier =
 
 @OptIn(ExperimentalSplitPaneApi::class)
 fun main() = singleWindowApplication(
+    state = WindowState(
+        size  = DpSize(800.dp, 700.dp)
+    ),
     title = "SplitPane demo"
 ) {
+
     MaterialTheme {
         val splitterState = rememberSplitPaneState()
         val hSplitterState = rememberSplitPaneState()
@@ -53,6 +59,13 @@ fun main() = singleWindowApplication(
                     Text(if (hSplitterState.moveEnabled) "Freeze H" else "Unfreeze H")
                 }
 
+                Button(onClick = { splitterState.firstVisible = !splitterState.firstVisible }) {
+                    Text(if (splitterState.firstVisible) "Hide V First" else "Show V First")
+                }
+                Button(onClick = { hSplitterState.firstVisible = !hSplitterState.firstVisible }) {
+                    Text(if (hSplitterState.firstVisible) "Hide H First" else "Show H First")
+                }
+
                 OutlinedTextField(value = delta, onValueChange = { delta = it }, label = { Text("Delta") })
                 Button(onClick = { delta.toFloatOrNull()?.let { splitterState.dispatchRawMovement(it) } }) {
                     Text("Add delta V")

components/SplitPane/library/src/commonMain/kotlin/org/jetbrains/compose/splitpane/SplitPaneDSL.kt

@@ -183,11 +183,13 @@ internal class SplitPaneScopeImpl(
 @Composable
 fun rememberSplitPaneState(
     initialPositionPercentage: Float = 0f,
-    moveEnabled: Boolean = true
+    moveEnabled: Boolean = true,
+    firstVisible: Boolean = true
 ): SplitPaneState {
     return remember {
         SplitPaneState(
             moveEnabled = moveEnabled,
+            firstVisible = firstVisible,
             initialPositionPercentage = initialPositionPercentage
         )
     }

components/SplitPane/library/src/commonMain/kotlin/org/jetbrains/compose/splitpane/SplitPaneState.kt

@@ -8,10 +8,13 @@ import androidx.compose.runtime.setValue
 class SplitPaneState(
     initialPositionPercentage: Float,
     moveEnabled: Boolean,
+    firstVisible: Boolean = true
 ) {
 
     var moveEnabled by mutableStateOf(moveEnabled)
 
+    var firstVisible by mutableStateOf(firstVisible)
+
     private var _positionPercentage by mutableStateOf(initialPositionPercentage)
     var positionPercentage: Float
         get() = _positionPercentage

components/SplitPane/library/src/desktopMain/kotlin/org/jetbrains/compose/splitpane/DesktopSplitPane.kt

@@ -14,8 +14,8 @@ private fun Constraints.withUnconstrainedWidth() = copy(minWidth = 0, maxWidth =
 private fun Constraints.withUnconstrainedHeight() = copy(minHeight = 0, maxHeight = Constraints.Infinity)
 
 
-@OptIn(ExperimentalSplitPaneApi::class)
 @Composable
+@OptIn(ExperimentalSplitPaneApi::class)
 internal actual fun SplitPane(
     modifier: Modifier,
     isHorizontal: Boolean,
@@ -30,7 +30,7 @@ internal actual fun SplitPane(
         second?.let { Box(modifier) { it() } }
         return
     }
-    
+
     Layout(
         {
             Box {
@@ -77,8 +77,10 @@ internal actual fun SplitPane(
                 minPosition = constrainedMin
                 maxPosition = constrainedMax
 
-                val position = (constrainedMin * (1-positionPercentage) + constrainedMax * positionPercentage)
-                    .roundToInt()
+                val position =
+                    if (firstVisible) {
+                        (constrainedMin * (1 - positionPercentage) + constrainedMax * positionPercentage).roundToInt()
+                    } else 0
 
                 val firstPlaceable = firstMeasurable.measure(
                     if (isHorizontal) {
@@ -94,7 +96,7 @@ internal actual fun SplitPane(
                     }
                 )
 
-                val secondPlaceablePosition = position + splitterSize
+                val secondPlaceablePosition = if (firstVisible) position + splitterSize else 0
                 val secondAvailableSize = (maxConstraintOnMainAxis - secondPlaceablePosition).coerceAtLeast(0)
 
                 val secondPlaceable = secondMeasurable.measure(