Bug 1823284 - Avoid waiting for window resizing if it is already at e…

…xpected dimensions; r=smaug

Depends on D175384

Differential Revision: https://phabricator.services.mozilla.com/D175213

xpfe/appshell/AppWindow.cpp

@@ -2946,7 +2946,23 @@ void AppWindow::FullscreenWillChange(bool aInFullscreen) {
     }
   }
   MOZ_ASSERT(mFullscreenChangeState == FullscreenChangeState::NotChanging);
-  mFullscreenChangeState = FullscreenChangeState::WillChange;
+
+  int32_t winWidth = 0;
+  int32_t winHeight = 0;
+  GetSize(&winWidth, &winHeight);
+
+  int32_t screenWidth = 0;
+  int32_t screenHeight = 0;
+  GetAvailScreenSize(&screenWidth, &screenHeight);
+
+  // Check if the window is already at the expected dimensions. If it is, set
+  // the fullscreen change state to WidgetResized to avoid waiting for a resize
+  // event. On macOS, a fullscreen window could be slightly higher than
+  // available screen size because of the OS menu bar isn't yet hidden.
+  mFullscreenChangeState =
+      (aInFullscreen == (winWidth == screenWidth && winHeight >= screenHeight))
+          ? FullscreenChangeState::WidgetResized
+          : FullscreenChangeState::WillChange;
 }
 
 void AppWindow::FullscreenChanged(bool aInFullscreen) {