Bug 1465616 - Use layout viewport size to compute visible rect for fixed position elements. r=mstange

Since fixed position elements are now scrollable, we need to ensure that they're
drawn using the layout viewport size instead of only the SPC-SPS, since
otherwise they'd be inaccurately clipped when scrolled.

MozReview-Commit-ID: 4p3pWnwluvz

--HG--
extra : rebase_source : f5bc1eae9bf2c8f4b9f78675e9da071de012160e

layout/painting/nsDisplayList.h

@@ -1462,24 +1462,18 @@ public:
       nsRect visible = aVisibleRect;
       nsRect dirtyRectRelativeToDirtyFrame = aDirtyRect;
 
+#ifdef MOZ_WIDGET_ANDROID
       if (nsLayoutUtils::IsFixedPosFrameInDisplayPort(aFrame) &&
           aBuilder->IsPaintingToWindow()) {
-        // position: fixed items are reflowed into and only drawn inside the
-        // viewport, or the scroll position clamping scrollport size, if one is
-        // set.
         nsIPresShell* ps = aFrame->PresShell();
-        if (ps->IsScrollPositionClampingScrollPortSizeSet()) {
-          dirtyRectRelativeToDirtyFrame =
-            nsRect(nsPoint(0, 0), ps->GetScrollPositionClampingScrollPortSize());
-          visible = dirtyRectRelativeToDirtyFrame;
-#ifdef MOZ_WIDGET_ANDROID
-        } else {
+        if (!ps->IsScrollPositionClampingScrollPortSizeSet()) {
           dirtyRectRelativeToDirtyFrame =
             nsRect(nsPoint(0, 0), aFrame->GetParent()->GetSize());
           visible = dirtyRectRelativeToDirtyFrame;
-#endif
         }
       }
+#endif
+
       *aOutDirtyRect = dirtyRectRelativeToDirtyFrame - aFrame->GetPosition();
       visible -= aFrame->GetPosition();