Bug 1493976 - Reset the resolution to 1 when entering fullscreen mode. r=kats,xidorn

The previous resolution is restored when exiting fullscreen mode.

Depends on D9442

Differential Revision: https://phabricator.services.mozilla.com/D9443

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

dom/base/nsDocument.cpp

@@ -1510,7 +1510,8 @@ nsIDocument::nsIDocument()
     mIgnoreOpensDuringUnloadCounter(0),
     mNumTrackersFound(0),
     mNumTrackersBlocked(0),
-    mDocLWTheme(Doc_Theme_Uninitialized)
+    mDocLWTheme(Doc_Theme_Uninitialized),
+    mSavedResolution(1.0f)
 {
   SetIsInDocument();
   SetIsConnected(true);
@@ -11186,6 +11187,16 @@ nsIDocument::CleanupFullscreenState()
   }
   mFullscreenStack.Clear();
   mFullscreenRoot = nullptr;
+
+  // Restore the zoom level that was in place prior to entering fullscreen.
+  if (nsIPresShell* shell = GetShell()) {
+    if (nsPresContext* context = shell->GetPresContext()) {
+      if (context->IsRootContentDocument()) {
+        shell->SetResolutionAndScaleTo(mSavedResolution);
+      }
+    }
+  }
+
   UpdateViewportScrollbarOverrideForFullscreen(this);
 }
 
@@ -11589,6 +11600,23 @@ nsIDocument::ApplyFullscreen(UniquePtr<FullscreenRequest> aRequest)
   nsIDocument* child = this;
   while (true) {
     child->SetFullscreenRoot(fullScreenRootDoc);
+
+    // When entering fullscreen, reset the RCD's zoom level to 1,
+    // otherwise the fullscreen content could be sized larger than the
+    // screen (since fullscreen is implemented using position:fixed and
+    // fixed elements are sized to the layout viewport).
+    // This also ensures that things like video controls aren't zoomed in
+    // when in fullscreen mode.
+    if (nsIPresShell* shell = child->GetShell()) {
+      if (nsPresContext* context = shell->GetPresContext()) {
+        if (context->IsRootContentDocument()) {
+          // Save the previous resolution so it can be restored.
+          child->mSavedResolution = shell->GetResolution();
+          shell->SetResolutionAndScaleTo(1.0f);
+        }
+      }
+    }
+
     NS_ASSERTION(child->GetFullscreenRoot() == fullScreenRootDoc,
         "Fullscreen root should be set!");
     if (child == fullScreenRootDoc) {

dom/base/nsIDocument.h

@@ -4771,6 +4771,9 @@ protected:
   // document lightweight theme for use with :-moz-lwtheme, :-moz-lwtheme-brighttext
   // and :-moz-lwtheme-darktext
   DocumentTheme                         mDocLWTheme;
+
+  // Pres shell resolution saved before entering fullscreen mode.
+  float mSavedResolution;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIDocument, NS_IDOCUMENT_IID)