Fix split panel (#2175)

* Fix ignoring modifier when one of the parts is missing

* Fix ignoring density for min size when measuring

* Fix a copypasta for secondMinSizePx

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

@@ -28,20 +28,15 @@ fun VerticalSplitPane(
     content: SplitPaneScope.() -> Unit
 ) {
     with(SplitPaneScopeImpl(isHorizontal = false, splitPaneState).apply(content)) {
-        if (firstPlaceableContent != null && secondPlaceableContent != null) {
         SplitPane(
             modifier = modifier,
             isHorizontal = false,
             splitPaneState = splitPaneState,
             minimalSizesConfiguration = minimalSizes,
-                first = firstPlaceableContent!!,
-                second = secondPlaceableContent!!,
+            first = firstPlaceableContent,
+            second = secondPlaceableContent,
             splitter = splitter
         )
-        } else {
-            firstPlaceableContent?.invoke()
-            secondPlaceableContent?.invoke()
-        }
     }
 }
 
@@ -63,7 +58,6 @@ fun HorizontalSplitPane(
     content: SplitPaneScope.() -> Unit
 ) {
     with(SplitPaneScopeImpl(isHorizontal = true, splitPaneState).apply(content)) {
-        if (firstPlaceableContent != null && secondPlaceableContent != null) {
         SplitPane(
             modifier = modifier,
             isHorizontal = true,
@@ -73,10 +67,6 @@ fun HorizontalSplitPane(
             second = secondPlaceableContent!!,
             splitter = splitter
         )
-        } else {
-            firstPlaceableContent?.invoke()
-            secondPlaceableContent?.invoke()
-        }
     }
 
 }
@@ -105,12 +95,13 @@ internal expect fun defaultSplitter(
  * @param splitter separator composable, by default [Splitter] is used
  * */
 @Composable
+@OptIn(ExperimentalSplitPaneApi::class)
 internal expect fun SplitPane(
     modifier: Modifier = Modifier,
     isHorizontal: Boolean = true,
     splitPaneState: SplitPaneState,
     minimalSizesConfiguration: MinimalSizes = MinimalSizes(0.dp, 0.dp),
-    first: @Composable () -> Unit,
-    second: @Composable () -> Unit,
+    first: (@Composable () -> Unit)?,
+    second: (@Composable () -> Unit)?,
     splitter: Splitter
 )

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

@@ -19,10 +19,16 @@ internal actual fun SplitPane(
     isHorizontal: Boolean,
     splitPaneState: SplitPaneState,
     minimalSizesConfiguration: MinimalSizes,
-    first: @Composable () -> Unit,
-    second: @Composable () -> Unit,
+    first: (@Composable () -> Unit)?,
+    second: (@Composable () -> Unit)?,
     splitter: Splitter
 ) {
+    if (first == null || second == null) {
+        first?.let { Box(modifier) { it() } }
+        second?.let { Box(modifier) { it() } }
+        return
+    }
+    
     Layout(
         {
             Box {
@@ -41,12 +47,13 @@ internal actual fun SplitPane(
         modifier,
     ) { measurables, constraints ->
         with(minimalSizesConfiguration) {
-            with(splitPaneState) {
-
-                val constrainedMin = constraints.minByDirection(isHorizontal) + firstPlaceableMinimalSize.value
+            val firstMinSizePx = firstPlaceableMinimalSize.value * density
+            val secondMinSizePx = secondPlaceableMinimalSize.value * density
 
+            with(splitPaneState) {
+                val constrainedMin = constraints.minByDirection(isHorizontal) + firstMinSizePx
                 val constrainedMax =
-                    (constraints.maxByDirection(isHorizontal).toFloat() - secondPlaceableMinimalSize.value).let {
+                    (constraints.maxByDirection(isHorizontal).toFloat() - secondMinSizePx).let {
                         if (it <= 0 || it <= constrainedMin) {
                             constraints.maxByDirection(isHorizontal).toFloat()
                         } else {
@@ -60,7 +67,7 @@ internal actual fun SplitPane(
 
                 if (maxPosition != constrainedMax) {
                     maxPosition =
-                        if ((firstPlaceableMinimalSize + secondPlaceableMinimalSize).value < constraints.maxByDirection(isHorizontal)) {
+                        if (firstMinSizePx + secondMinSizePx < constraints.maxByDirection(isHorizontal)) {
                             constrainedMax
                         } else {
                             minPosition
@@ -68,7 +75,7 @@ internal actual fun SplitPane(
                 }
 
                 val constrainedPosition =
-                    (constraints.maxByDirection(isHorizontal) - (firstPlaceableMinimalSize + secondPlaceableMinimalSize).value).let {
+                    (constraints.maxByDirection(isHorizontal) - (firstMinSizePx + secondMinSizePx)).let {
                         if (it > 0f) {
                             (it * positionPercentage).coerceIn(constrainedMin, constrainedMax).roundToInt()
                         } else {