Bug 1259641 - Do not force reflow for all tabs when size mode changed. r=smaug

MozReview-Commit-ID: GoTCM60Hmlf

--HG--
extra : transplant_source : S%88%AD%A3%99%9Bk%D3%E4U%0302%82%87o3%28%FFK

dom/ipc/TabChild.cpp

@@ -1655,13 +1655,15 @@ bool
 TabChild::RecvSizeModeChanged(const nsSizeMode& aSizeMode)
 {
   mPuppetWidget->SetSizeMode(aSizeMode);
+  if (!mPuppetWidget->IsVisible()) {
+    return true;
+  }
   nsCOMPtr<nsIDocument> document(GetDocument());
   nsCOMPtr<nsIPresShell> presShell = document->GetShell();
   if (presShell) {
     nsPresContext* presContext = presShell->GetPresContext();
     if (presContext) {
-      presContext->MediaFeatureValuesChangedAllDocuments(eRestyle_Subtree,
-                                                         NS_STYLE_HINT_REFLOW);
+      presContext->SizeModeChanged(aSizeMode);
     }
   }
   return true;

layout/style/nsMediaFeatures.cpp

@@ -328,6 +328,9 @@ GetDisplayMode(nsPresContext* aPresContext, const nsMediaFeature*,
   baseWindow->GetMainWidget(getter_AddRefs(mainWidget));
   int32_t displayMode;
   nsSizeMode mode = mainWidget ? mainWidget->SizeMode() : nsSizeMode_Normal;
+  // Background tabs are always in 'browser' mode for now.
+  // If new modes are supported, please ensure not cause the regression in
+  // Bug 1259641.
   switch (mode) {
     case nsSizeMode_Fullscreen:
       displayMode = NS_STYLE_DISPLAY_MODE_FULLSCREEN;