FlatLaf window decorations: fixed sometimes broken window moving with…

… SplitPane in window title area in "full window content" mode (issue #926)

CHANGELOG.md

@@ -14,6 +14,8 @@ FlatLaf Change Log
     factor. (issue #904)
   - Linux: Fixed continuous cursor toggling between resize and standard cursor
     when resizing window. (issue #907)
+  - Fixed sometimes broken window moving with SplitPane in window title area in
+    "full window content" mode. (issue #926)
 - Popup: On Windows 10, fixed misplaced popup drop shadow. (issue #911;
   regression in 3.5)
 - Popup: Fixed NPE if `GraphicsConfiguration` is `null` on Windows. (issue #921)

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTitlePane.java

@@ -1109,8 +1109,18 @@ private boolean isTitleBarCaptionAt( Component c, int x, int y ) {
 			// if component is not fully layouted, do not invoke function
 			// because it is too dangerous that the function tries to layout the component,
 			// which could cause a dead lock
-			if( !c.isValid() )
+			if( !c.isValid() ) {
+				// revalidate if necessary so that it is valid when invoked again later
+				EventQueue.invokeLater( () -> {
+					Window w = SwingUtilities.windowForComponent( c );
+					if( w != null )
+						w.revalidate();
+					else
+						c.revalidate();
+				} );
+
 				return false; // assume that this is not a caption because the component has mouse listeners
+			}
 
 			if( caption instanceof Function ) {
 				// check client property function value