Bug 1592933 - [Wayland] Cache scale factor for toplevel windows, r=jhorak

Differential Revision: https://phabricator.services.mozilla.com/D51253

--HG--
extra : moz-landing-system : lando

widget/gtk/nsWindow.cpp

@@ -476,6 +476,9 @@ nsWindow::nsWindow() {
 
   mHasAlphaVisual = false;
   mIsPIPWindow = false;
+
+  mWindowScaleFactorChanged = true;
+  mWindowScaleFactor = 1;
 }
 
 nsWindow::~nsWindow() {
@@ -3440,6 +3443,9 @@ void nsWindow::OnCompositedChanged() {
 }
 
 void nsWindow::OnScaleChanged(GtkAllocation* aAllocation) {
+  // Force scale factor recalculation
+  mWindowScaleFactorChanged = true;
+
   // This eventually propagate new scale to the PuppetWidgets
   OnDPIChanged();
 
@@ -6747,6 +6753,12 @@ GtkWindow* nsWindow::GetCurrentTopmostWindow() {
 }
 
 gint nsWindow::GdkScaleFactor() {
+  // We depend on notify::scale-factor callback which is reliable for toplevel
+  // windows only, so don't use scale cache for popup windows.
+  if (mWindowType == eWindowType_toplevel && !mWindowScaleFactorChanged) {
+    return mWindowScaleFactor;
+  }
+
   GdkWindow* scaledGdkWindow = mGdkWindow;
   if (!mIsX11Display) {
     // For popup windows/dialogs with parent window we need to get scale factor
@@ -6767,13 +6779,18 @@ gint nsWindow::GdkScaleFactor() {
       }
     }
   }
+
   // Available as of GTK 3.10+
   static auto sGdkWindowGetScaleFactorPtr =
       (gint(*)(GdkWindow*))dlsym(RTLD_DEFAULT, "gdk_window_get_scale_factor");
   if (sGdkWindowGetScaleFactorPtr && scaledGdkWindow) {
-    return (*sGdkWindowGetScaleFactorPtr)(scaledGdkWindow);
+    mWindowScaleFactor = (*sGdkWindowGetScaleFactorPtr)(scaledGdkWindow);
+  } else {
+    mWindowScaleFactor = ScreenHelperGTK::GetGTKMonitorScaleFactor();
   }
-  return ScreenHelperGTK::GetGTKMonitorScaleFactor();
+  mWindowScaleFactorChanged = false;
+
+  return mWindowScaleFactor;
 }
 
 gint nsWindow::DevicePixelsToGdkCoordRoundUp(int pixels) {

widget/gtk/nsWindow.h

@@ -448,6 +448,8 @@ class nsWindow final : public nsBaseWidget {
   bool mNeedsUpdatingEGLSurface;
   bool mCompositorInitiallyPaused;
 #endif
+  bool mWindowScaleFactorChanged;
+  int mWindowScaleFactor;
 
  private:
   void DestroyChildWindows();