firefox/D173814.diff

diff --git a/widget/gtk/MozContainerWayland.h b/widget/gtk/MozContainerWayland.h
--- a/widget/gtk/MozContainerWayland.h
+++ b/widget/gtk/MozContainerWayland.h
@@ -83,10 +83,13 @@
                                                         nsIntSize aSize,
                                                         int scale);
 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);
 void moz_container_wayland_add_or_fire_initial_draw_callback(
     MozContainer* container, const std::function<void(void)>& initial_draw_cb);
diff --git a/widget/gtk/MozContainerWayland.cpp b/widget/gtk/MozContainerWayland.cpp
--- a/widget/gtk/MozContainerWayland.cpp
+++ b/widget/gtk/MozContainerWayland.cpp
@@ -595,10 +595,17 @@
   if (container->wl_container.surface) {
     moz_container_wayland_set_scale_factor_locked(lock, 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;
 
   LOGWAYLAND("%s [%p]\n", __FUNCTION__,
diff --git a/widget/gtk/WindowSurfaceWaylandMultiBuffer.cpp b/widget/gtk/WindowSurfaceWaylandMultiBuffer.cpp
--- a/widget/gtk/WindowSurfaceWaylandMultiBuffer.cpp
+++ b/widget/gtk/WindowSurfaceWaylandMultiBuffer.cpp
@@ -283,12 +283,12 @@
     return;
   }
   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 "
         "wl_surface\n",
         (void*)mWindow.get());
@@ -317,12 +317,23 @@
       LayoutDeviceIntRect r = iter.Get();
       wl_surface_damage_buffer(waylandSurface, r.x, r.y, r.width, r.height);
     }
   }
 
+  // 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;
   mFrontBufferInvalidRegion = aInvalidRegion;
   mInProgressBuffer = nullptr;