Bug 417967. Align subframe drawing so that drawing at (0,0) relative to the root view's origin is aligned to a pixel boundary. r+sr=bzbarsky

layout/base/nsDisplayList.h

@@ -658,9 +658,10 @@ public:
   void OptimizeVisibility(nsDisplayListBuilder* aBuilder, nsRegion* aVisibleRegion);
   /**
    * Paint the list to the rendering context. We assume that (0,0) in aCtx
-   * corresponds to the origin of the reference frame. The rectangle in
+   * corresponds to the origin of the reference frame. For best results,
-   * aDirtyRect is painted, which *must* be contained in the dirty rect
+   * aCtx's current transform should make (0,0) pixel-aligned. The
-   * used to construct the display list.
+   * rectangle in aDirtyRect is painted, which *must* be contained in the
+   * dirty rect used to construct the display list.
    */
   void Paint(nsDisplayListBuilder* aBuilder, nsIRenderingContext* aCtx,
              const nsRect& aDirtyRect) const;

layout/base/nsLayoutUtils.h

@@ -394,7 +394,8 @@ public:
    * Given aFrame, the root frame of a stacking context, paint it and its
    * descendants to aRenderingContext. 
    * @param aRenderingContext a rendering context translated so that (0,0)
-   * is the origin of aFrame
+   * is the origin of aFrame; for best results, (0,0) should transform
+   * to pixel-aligned coordinates
    * @param aDirtyRegion the region that must be painted, in the coordinates
    * of aFrame
    * @param aBackground paint the dirty area with this color before drawing

view/public/nsIViewObserver.h

@@ -59,7 +59,10 @@ public:
   /* called when the observer needs to paint. This paints the entire
    * frame subtree rooted at the view, including frame subtrees from
    * subdocuments.
-   * @param aRenderingContext rendering context to paint to
+   * @param aRenderingContext rendering context to paint to; the origin
+   * of the view is painted at (0,0) in the rendering context's current
+   * transform. For best results this should transform to pixel-aligned
+   * coordinates.
    * @param aDirtyRegion the region to be painted, in the coordinates of aRootView
    * @return error status
    */

view/src/nsViewManager.cpp

@@ -597,8 +597,24 @@ void nsViewManager::RenderViews(nsView *aView, nsIRenderingContext& aRC,
     nsRegion damageRegion(aRegion);
     damageRegion.MoveBy(offsetToRoot);
 
+    gfxContext* ctx = aRC.ThebesContext();
+    nsCOMPtr<nsIDeviceContext> dc;
+    aRC.GetDeviceContext(*getter_AddRefs(dc));
+    double appPerDev = dc->AppUnitsPerDevPixel();
+    gfxRect r(-offsetToRoot.x/appPerDev, -offsetToRoot.y/appPerDev, 1.0, 1.0);
+
     aRC.PushState();
+    // Translate by the pixel-snapped offsetToRoot so that aRC's (0,0) will
+    // be aligned to pixel boundaries. We use gfx pixel-snapping here to
+    // ensure that the snapping we do here is consistent with other gfx
+    // snapping. For example if someone drew a border around the outside
+    // of aView, we want our (0,0) to be the inside top-left of that
+    // border.
+    if (ctx->UserToDevicePixelSnapped(r)) {
+      ctx->Translate(ctx->DeviceToUser(r).pos);
+    } else {
       aRC.Translate(-offsetToRoot.x, -offsetToRoot.y);
+    }
     mObserver->Paint(displayRoot, &aRC, damageRegion);
     aRC.PopState();
   }