Fix hidden title bar window dragging and resizing on macOS

- With TitleBarStyle::Hidden the system no longer moves the window for
  drags in the former title bar area, which it did even over content that
  handles the mouse. SetMovable()/IsMovable() keep the caller's preference;
  StartDragging() still works.
- Switching title bar styles keeps the window frame, as on Windows, instead
  of shifting the window by the title bar height.
- GetTitleBarStyle() reads the state from the NSWindow, so every wrapper of
  the same window agrees.

Author: lijy91

src/platform/macos/window_macos.mm

@@ -19,6 +19,32 @@
 // need to read the focusable flag without access to the Impl.
 static const void* kWindowFocusableKey = &kWindowFocusableKey;
 static const void* kWindowOriginalClassKey = &kWindowOriginalClassKey;
+// Window::SetTitleBarStyle() / SetMovable() state, kept on the NSWindow for the same reason.
+static const void* kWindowTitleBarHiddenKey = &kWindowTitleBarHiddenKey;
+static const void* kWindowMovableKey = &kWindowMovableKey;
+
+static BOOL NativeApiWindowIsTitleBarHidden(NSWindow* window) {
+  return [objc_getAssociatedObject(window, kWindowTitleBarHiddenKey) boolValue];
+}
+
+// Whether the user may move the window, as requested through Window::SetMovable().
+static BOOL NativeApiWindowIsMovable(NSWindow* window) {
+  NSNumber* value = objc_getAssociatedObject(window, kWindowMovableKey);
+  return value ? [value boolValue] : [window isMovable];
+}
+
+// With the title bar hidden, the content extends under the title bar band, but AppKit still
+// moves the window for drags that start in that band, even over opaque content views that handle
+// the mouse themselves (the window server decides from its own drag region, not from
+// -mouseDownCanMoveWindow). The content owns the band then, so the system is kept from moving
+// the window; Window::StartDragging() (-performWindowDragWithEvent:) and SetPosition() still
+// work, and are how apps with a custom title bar move it.
+static void NativeApiUpdateWindowMovable(NSWindow* window) {
+  BOOL requested = NativeApiWindowIsMovable(window);
+  objc_setAssociatedObject(window, kWindowMovableKey, @(requested),
+                           OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+  [window setMovable:requested && !NativeApiWindowIsTitleBarHidden(window)];
+}
 
 static BOOL NativeApiWindowIsFocusable(NSWindow* window) {
   NSNumber* value = objc_getAssociatedObject(window, kWindowFocusableKey);
@@ -108,12 +134,10 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
   Impl(WindowId id, NSWindow* window)
       : id_(id),
         ns_window_(window),
-        title_bar_style_(TitleBarStyle::Normal),
         visual_effect_(VisualEffect::None),
         visual_effect_view_(nil) {}
   WindowId id_;
   NSWindow* ns_window_;
-  TitleBarStyle title_bar_style_;
   VisualEffect visual_effect_;
   NSVisualEffectView* visual_effect_view_;
   double aspect_ratio_ = 0.0;
@@ -358,11 +382,18 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
 }
 
 void Window::SetMovable(bool is_movable) {
-  [pimpl_->ns_window_ setMovable:is_movable];
+  NSWindow* window = pimpl_->ns_window_;
+  if (!window) {
+    return;
+  }
+  objc_setAssociatedObject(window, kWindowMovableKey, @(is_movable),
+                           OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+  NativeApiUpdateWindowMovable(window);
 }
 
 bool Window::IsMovable() const {
-  return [pimpl_->ns_window_ isMovable];
+  NSWindow* window = pimpl_->ns_window_;
+  return window && NativeApiWindowIsMovable(window);
 }
 
 void Window::SetMinimizable(bool is_minimizable) {
@@ -499,7 +530,20 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
 }
 
 void Window::SetTitleBarStyle(TitleBarStyle style) {
-  pimpl_->title_bar_style_ = style;
+  if (!pimpl_->ns_window_) {
+    return;
+  }
+  // Record the movable preference before the hidden flag starts to affect -isMovable.
+  NativeApiUpdateWindowMovable(pimpl_->ns_window_);
+  objc_setAssociatedObject(pimpl_->ns_window_, kWindowTitleBarHiddenKey,
+                           @(style == TitleBarStyle::Hidden), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+  NativeApiUpdateWindowMovable(pimpl_->ns_window_);
+
+  // Changing NSWindowStyleMaskFullSizeContentView keeps the content size and the bottom-left
+  // origin, so the window would grow or shrink by the title bar height and its top edge would
+  // jump. Keep the frame instead, as on Windows: the content takes over, or gives back, the title
+  // bar area.
+  const NSRect frame = pimpl_->ns_window_.frame;
 
   if (style == TitleBarStyle::Hidden) {
     // Hide title bar - make it transparent and full size content view
@@ -512,6 +556,9 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
     pimpl_->ns_window_.titlebarAppearsTransparent = NO;
     pimpl_->ns_window_.styleMask &= ~NSWindowStyleMaskFullSizeContentView;
   }
+  if (!NSEqualRects(pimpl_->ns_window_.frame, frame)) {
+    [pimpl_->ns_window_ setFrame:frame display:YES];
+  }
 
   // Ensure window remains opaque and has shadow
   pimpl_->ns_window_.opaque = NO;
@@ -530,7 +577,8 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
 }
 
 TitleBarStyle Window::GetTitleBarStyle() const {
-  return pimpl_->title_bar_style_;
+  return NativeApiWindowIsTitleBarHidden(pimpl_->ns_window_) ? TitleBarStyle::Hidden
+                                                             : TitleBarStyle::Normal;
 }
 
 void Window::SetHasShadow(bool has_shadow) {

src/window.h

@@ -40,6 +40,16 @@ enum class TitleBarStyle {
   /**
    * Hidden title bar with no visible decorations.
    * The window appears without a title bar, useful for custom chrome.
+   *
+   * The content owns the area where the title bar was: dragging there does
+   * not move the window. Move it from custom chrome with
+   * Window::StartDragging() (or a WindowDragSession).
+   * - macOS: the content extends under a transparent title bar; the
+   *   window buttons stay. The system is kept from moving the window, even
+   *   while IsMovable() is true.
+   *
+   * Switching between styles keeps the window's frame (position and outer
+   * size); the content area grows or shrinks by the title bar instead.
    */
   Hidden
 };
@@ -439,6 +449,10 @@ class Window : public NativeObjectProvider, public std::enable_shared_from_this<
    *
    * When disabled, the user cannot move the window by dragging its title bar.
    * Programmatic positioning via SetPosition() is still possible.
+   *
+   * With TitleBarStyle::Hidden the system does not move the window on its
+   * own regardless; this setting is kept and applies again once the title
+   * bar is shown.
    */
   void SetMovable(bool is_movable);