Bug 1169879 - Use only the critical displayport when computing image visibility. r=tn

layout/base/nsLayoutUtils.cpp

@@ -1065,6 +1065,13 @@ nsLayoutUtils::GetDisplayPort(nsIContent* aContent, nsRect *aResult)
   return GetDisplayPortImpl(aContent, aResult, 1.0f);
 }
 
+/* static */ bool
+nsLayoutUtils::GetDisplayPortForVisibilityTesting(nsIContent* aContent,
+                                                  nsRect* aResult)
+{
+  return GetDisplayPortImpl(aContent, aResult, 1.0f);
+}
+
 bool
 nsLayoutUtils::SetDisplayPortMargins(nsIContent* aContent,
                                      nsIPresShell* aPresShell,

layout/base/nsLayoutUtils.h

@@ -165,6 +165,16 @@ public:
    */
   static bool GetDisplayPort(nsIContent* aContent, nsRect *aResult = nullptr);
 
+  /**
+   * @return the display port for the given element which should be used for
+   * visibility testing purposes.
+   *
+   * If low-precision buffers are enabled, this is the critical display port;
+   * otherwise, it's the same display port returned by GetDisplayPort().
+   */
+  static bool GetDisplayPortForVisibilityTesting(nsIContent* aContent,
+                                                 nsRect* aResult = nullptr);
+
   enum class RepaintMode : uint8_t {
     Repaint,
     DoNotRepaint

layout/base/nsPresShell.cpp

@@ -5940,7 +5940,9 @@ PresShell::MarkImagesInSubtreeVisible(nsIFrame* aFrame, const nsRect& aRect)
   nsIScrollableFrame* scrollFrame = do_QueryFrame(aFrame);
   if (scrollFrame) {
     nsRect displayPort;
-    bool usingDisplayport = nsLayoutUtils::GetDisplayPort(aFrame->GetContent(), &displayPort);
+    bool usingDisplayport =
+      nsLayoutUtils::GetDisplayPortForVisibilityTesting(aFrame->GetContent(),
+                                                        &displayPort);
     if (usingDisplayport) {
       rect = displayPort;
     } else {