Properly handle toggling decorations on Windows.

core/src/main/java/com/github/weisj/darklaf/ui/rootpane/DarkRootPaneUI.java

@@ -103,14 +103,15 @@ public class DarkRootPaneUI extends BasicRootPaneUI implements HierarchyListener
     public void propertyChange(final PropertyChangeEvent e) {
         super.propertyChange(e);
         String propertyName = e.getPropertyName();
-        if (PropertyKey.ANCESTOR.equals(propertyName)) {
+        if (PropertyKey.ANCESTOR.equals(propertyName) || KEY_NO_DECORATIONS.equals(propertyName)) {
             updateWindow(rootPane.getParent());
         }
     }
 
     protected int decorationsStyleFromWindow(final Window window, final int windowDecorationsStyle) {
+        if (DarkUIUtil.isUndecorated(window)
+            || PropertyUtil.getBooleanProperty(rootPane, KEY_NO_DECORATIONS)) return JRootPane.NONE;
         if (windowDecorationsStyle != JRootPane.NONE) return windowDecorationsStyle;
-        if (DarkUIUtil.isUndecorated(window)) return JRootPane.NONE;
         if (window instanceof JFrame) return JRootPane.FRAME;
         if (window instanceof JDialog) return JRootPane.PLAIN_DIALOG;
         return windowDecorationsStyle;
@@ -183,6 +184,7 @@ public class DarkRootPaneUI extends BasicRootPaneUI implements HierarchyListener
     private void updateWindow(final Component parent) {
         window = DarkUIUtil.getWindow(parent);
         windowDecorationsStyle = decorationsStyleFromWindow(window, windowDecorationsStyle);
+        if (titlePane != null) titlePane.setDecorationsStyle(windowDecorationsStyle);
         installBorder(rootPane);
     }
 

core/src/test/java/ui/ComponentDemo.java

@@ -206,6 +206,12 @@ public interface ComponentDemo {
         dev.add(logging);
         dev.add(aaPainting);
         dev.add(experimentalAA);
+        dev.add(new JCheckBoxMenuItem("Custom Decorations") {
+            {
+                setSelected(LafManager.isDecorationsEnabled());
+                addActionListener(e -> LafManager.setDecorationsEnabled(isSelected()));
+            }
+        });
         return dev;
     }
 

windows/src/main/cpp/Decorations.cpp

@@ -303,6 +303,28 @@ bool InstallDecorations(HWND handle, bool is_popup) {
     return true;
 }
 
+void UninstallDecorations(WindowWrapper *wrapper, bool decorated) {
+    HWND handle = wrapper->window;
+
+    // Restore old window procedure.
+    SetWindowLongPtr(handle, GWLP_WNDPROC, reinterpret_cast<LONG_PTR>(wrapper->prev_proc));
+
+    // Restore old client area.
+    MARGINS margins = { 0, 0, 0, 0 };
+    DwmExtendFrameIntoClientArea(handle, &margins);
+
+    auto style = GetWindowLongPtr(handle, GWL_STYLE);
+    if (!decorated) {
+        style &= ~WS_THICKFRAME;
+    } else {
+        style |= WS_THICKFRAME;
+    }
+    SetWindowLongPtr(handle, GWL_STYLE, style);
+
+    UINT flags = SWP_NOZORDER | SWP_NOOWNERZORDER | SWP_NOMOVE | SWP_NOSIZE | SWP_FRAMECHANGED;
+    SetWindowPos(handle, NULL, 0, 0, 0, 0, flags);
+}
+
 // @formatter:off
 LRESULT CALLBACK WindowWrapper::WindowProc(_In_ HWND hwnd, _In_ UINT uMsg, _In_ WPARAM wParam, _In_ LPARAM lParam) {
     HWND handle = reinterpret_cast<HWND>(hwnd);
@@ -402,11 +424,11 @@ Java_com_github_weisj_darklaf_platform_windows_JNIDecorationsWindows_installDeco
 }
 
 JNIEXPORT void JNICALL
-Java_com_github_weisj_darklaf_platform_windows_JNIDecorationsWindows_uninstallDecorations(JNIEnv *env, jclass obj, jlong hwnd) {
+Java_com_github_weisj_darklaf_platform_windows_JNIDecorationsWindows_uninstallDecorations(JNIEnv *env, jclass obj, jlong hwnd, jboolean decorated) {
     HWND handle = reinterpret_cast<HWND>(hwnd);
     auto wrap = wrapper_map[handle];
     if (wrap) {
-        SetWindowLongPtr(handle, GWLP_WNDPROC, reinterpret_cast<LONG_PTR>(wrap->prev_proc));
+        UninstallDecorations(wrap, decorated);
         wrapper_map.erase(handle);
         delete (wrap);
     }

windows/src/main/java/com/github/weisj/darklaf/platform/windows/JNIDecorationsWindows.java

@@ -43,7 +43,7 @@ public class JNIDecorationsWindows {
 
     public static native boolean installDecorations(final long hwnd);
 
-    public static native void uninstallDecorations(final long hwnd);
+    public static native void uninstallDecorations(final long hwnd, final boolean decorated);
 
     public static native boolean installPopupMenuDecorations(final long hwnd);
 }

windows/src/main/java/com/github/weisj/darklaf/platform/windows/WindowsDecorationsProvider.java

@@ -76,7 +76,7 @@ public class WindowsDecorationsProvider implements DecorationsProvider {
         if (window.isDisplayable()) {
             long hwnd = PointerUtil.getHWND(window);
             if (hwnd != 0) {
-                JNIDecorationsWindows.uninstallDecorations(hwnd);
+                JNIDecorationsWindows.uninstallDecorations(hwnd, false);
             }
             window.dispose();
         }

windows/src/main/java/com/github/weisj/darklaf/platform/windows/ui/WindowsTitlePane.java

@@ -130,7 +130,7 @@ public class WindowsTitlePane extends CustomTitlePane {
         this.rootPane = root;
         this.window = window;
         bundle = ResourceBundle.getBundle("com.github.weisj.darklaf.bundles.actions", getLocale());
-        setDecorationsStyle(decorationStyle);
+        this.decorationStyle = decorationStyle;
         rootPane.addContainerListener(rootPaneContainerListener);
         rootPane.getLayeredPane().addContainerListener(layeredPaneContainerListener);
         state = -1;
@@ -194,7 +194,7 @@ public class WindowsTitlePane extends CustomTitlePane {
 
     protected void uninstallDecorations() {
         if (windowHandle != 0) {
-            JNIDecorationsWindows.uninstallDecorations(windowHandle);
+            JNIDecorationsWindows.uninstallDecorations(windowHandle, decorationStyle != JRootPane.NONE);
             windowHandle = 0;
         }
         rootPane.removeContainerListener(rootPaneContainerListener);
@@ -224,7 +224,19 @@ public class WindowsTitlePane extends CustomTitlePane {
         }
     }
 
+    @Override
+    public void setDecorationsStyle(final int style) {
+        super.setDecorationsStyle(style);
+        if (style == JRootPane.NONE && windowHandle != 0) {
+            uninstall();
+        } else if (windowHandle == 0) {
+            install();
+        }
+    }
+
     private boolean installDecorations() {
+        if (getDecorationStyle() == JRootPane.NONE) return false;
+        if (!window.isDisplayable()) return false;
         if (window instanceof Dialog || window instanceof Frame) {
             windowHandle = PointerUtil.getHWND(window);
             if (windowHandle != 0) {