Bug 1803016 [Wayland] Don't commit wl_buffer if buffer scale doesn't match its size r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D173814

widget/gtk/MozContainerWayland.cpp

@@ -597,6 +597,13 @@ void moz_container_wayland_set_scale_factor(MozContainer* container) {
   }
 }
 
+bool moz_container_wayland_size_matches_scale_factor_locked(
+    const MutexAutoLock& aProofOfLock, MozContainer* container, int aWidth,
+    int aHeight) {
+  return aWidth % container->wl_container.buffer_scale == 0 &&
+         aHeight % container->wl_container.buffer_scale == 0;
+}
+
 static bool moz_container_wayland_surface_create_locked(
     const MutexAutoLock& aProofOfLock, MozContainer* container) {
   MozContainerWayland* wl_container = &container->wl_container;

widget/gtk/MozContainerWayland.h

@@ -85,6 +85,9 @@ bool moz_container_wayland_egl_window_needs_size_update(MozContainer* container,
 void moz_container_wayland_set_scale_factor(MozContainer* container);
 void moz_container_wayland_set_scale_factor_locked(
     const mozilla::MutexAutoLock& aProofOfLock, MozContainer* container);
+bool moz_container_wayland_size_matches_scale_factor_locked(
+    const mozilla::MutexAutoLock& aProofOfLock, MozContainer* container,
+    int aWidth, int aHeight);
 
 void moz_container_wayland_add_initial_draw_callback_locked(
     MozContainer* container, const std::function<void(void)>& initial_draw_cb);

widget/gtk/WindowSurfaceWaylandMultiBuffer.cpp

@@ -285,8 +285,8 @@ void WindowSurfaceWaylandMB::Commit(
   mFrameInProcess = false;
 
   MozContainer* container = mWindow->GetMozContainer();
-  MozContainerSurfaceLock lock(container);
-  struct wl_surface* waylandSurface = lock.GetSurface();
+  MozContainerSurfaceLock MozContainerLock(container);
+  struct wl_surface* waylandSurface = MozContainerLock.GetSurface();
   if (!waylandSurface) {
     LOGWAYLAND(
         "WindowSurfaceWaylandMB::Commit [%p] frame queued: can't lock "
@@ -319,8 +319,19 @@ void WindowSurfaceWaylandMB::Commit(
     }
   }
 
+  // aProofOfLock is a kind of substitution of MozContainerSurfaceLock.
+  // MozContainer is locked but MozContainerSurfaceLock doen't convert to
+  // MutexAutoLock& so we use aProofOfLock here.
   moz_container_wayland_set_scale_factor_locked(aProofOfLock, container);
-  mInProgressBuffer->AttachAndCommit(waylandSurface);
+
+  // It's possible that scale factor changed between Lock() and Commit()
+  // but window size is the same.
+  // Don't attach such buffer as it may have incorrect size,
+  // we'll paint new content soon.
+  if (moz_container_wayland_size_matches_scale_factor_locked(
+          aProofOfLock, container, mWindowSize.width, mWindowSize.height)) {
+    mInProgressBuffer->AttachAndCommit(waylandSurface);
+  }
 
   mInProgressBuffer->ResetBufferAge();
   mFrontBuffer = mInProgressBuffer;