Bug 994816 - Avoid doing any alignment of display port when tiling is disabled. r=tn

--HG--
extra : rebase_source : 39a12744d84990a395f41fcd53713337203e5666

layout/base/nsLayoutUtils.cpp

@@ -735,6 +735,14 @@ nsLayoutUtils::GetDisplayPort(nsIContent* aContent, nsRect *aResult)
         nsPoint scrollPos(
           scrollableFrame ? scrollableFrame->GetScrollPosition() : nsPoint(0,0));
         if (marginsData->mAlignmentX > 0 || marginsData->mAlignmentY > 0) {
+          // Avoid division by zero.
+          if (marginsData->mAlignmentX == 0) {
+            marginsData->mAlignmentX = 1;
+          }
+          if (marginsData->mAlignmentY == 0) {
+            marginsData->mAlignmentY = 1;
+          }
+
           LayerPoint scrollPosLayer(
             res.width * NSAppUnitsToFloatPixels(scrollPos.x, auPerDevPixel),
             res.height * NSAppUnitsToFloatPixels(scrollPos.y, auPerDevPixel));
@@ -2533,7 +2541,7 @@ nsLayoutUtils::GetOrMaybeCreateDisplayPort(nsDisplayListBuilder& aBuilder,
       nsIPresShell* presShell = aScrollFrame->PresContext()->GetPresShell();
       gfx::IntSize alignment = gfxPrefs::LayersTilesEnabled()
           ? gfx::IntSize(gfxPrefs::LayersTileWidth(), gfxPrefs::LayersTileHeight()) :
-            gfx::IntSize(1, 1);
+            gfx::IntSize(0, 0);
       nsLayoutUtils::SetDisplayPortMargins(
           content, presShell, displayportMargins, alignment.width,
           alignment.height, 0, nsLayoutUtils::RepaintMode::DoNotRepaint);

widget/xpwidgets/APZCCallbackHelper.cpp

@@ -230,7 +230,7 @@ APZCCallbackHelper::UpdateRootFrame(nsIDOMWindowUtils* aUtils,
     } else {
         gfx::IntSize alignment = gfxPrefs::LayersTilesEnabled()
             ? gfx::IntSize(gfxPrefs::LayersTileWidth(), gfxPrefs::LayersTileHeight()) :
-              gfx::IntSize(1, 1);
+              gfx::IntSize(0, 0);
         LayerMargin margins = aMetrics.GetDisplayPortMargins();
         aUtils->SetDisplayPortMarginsForElement(margins.left,
                                                 margins.top,
@@ -285,7 +285,7 @@ APZCCallbackHelper::UpdateSubFrame(nsIContent* aContent,
         } else {
             gfx::IntSize alignment = gfxPrefs::LayersTilesEnabled()
                 ? gfx::IntSize(gfxPrefs::LayersTileWidth(), gfxPrefs::LayersTileHeight()) :
-                  gfx::IntSize(1, 1);
+                  gfx::IntSize(0, 0);
             LayerMargin margins = aMetrics.GetDisplayPortMargins();
             utils->SetDisplayPortMarginsForElement(margins.left,
                                                    margins.top,