Bug 556052. Compute mViewToWidgetOffset correctly and fix bugs by adding subpixel translation when painting. r=mats

layout/base/nsPresShell.cpp

@@ -5655,8 +5655,11 @@ PresShell::Paint(nsIView*        aDisplayRoot,
           nsPoint offsetToRoot = aViewToPaint->GetOffsetTo(aDisplayRoot);
           nsRegion dirtyRegion = aDirtyRegion;
           dirtyRegion.MoveBy(offsetToRoot);
+
+          nsPoint translate = -offsetToRoot + aViewToPaint->ViewToWidgetOffset();
           nsIRenderingContext::AutoPushTranslation
-            push(rc, -offsetToRoot.x, -offsetToRoot.y);
+            push(rc, translate.x, translate.y);
+
           nsLayoutUtils::PaintFrame(rc, frame, dirtyRegion, bgcolor);
         }
       } else {

view/public/nsIView.h

@@ -339,6 +339,24 @@ public:
 
   PRBool IsEffectivelyVisible();
 
+  // This is an app unit offset to add when converting view coordinates to
+  // widget coordinates.  It is the offset in view coordinates from widget
+  // top-left to view top-left.
+  nsPoint ViewToWidgetOffset() const {
+    nsIView* parent = reinterpret_cast<nsIView*>(mParent);
+    if (parent && parent->GetViewManager() != GetViewManager()) {
+      // The document root view's mViewToWidgetOffset is always (0,0).
+      // If it has a parent view, the parent view must be the inner view
+      // for an nsSubdocumentFrame; its top-left position in appunits
+      // is always positioned at that inner view's top-left, and its
+      // widget top-left is always positioned at that inner view's widget's
+      // top-left, so its ViewToWidgetOffset is actually the same as
+      // its parent's.
+      return parent->ViewToWidgetOffset();
+    }
+    return mViewToWidgetOffset;
+  }
+
 protected:
   friend class nsWeakView;
   nsViewManager     *mViewManager;

view/src/nsView.cpp

@@ -347,25 +347,35 @@ nsIntRect nsIView::CalcWidgetBounds(nsWindowType aType)
   nsRect viewBounds(mDimBounds);
 
   if (GetParent()) {
-    // put offset into screen coordinates
     nsPoint offset;
     nsIWidget* parentWidget = GetParent()->GetNearestWidget(&offset);
+    // make viewBounds be relative to the parent widget, in appunits
     viewBounds += offset;
 
     if (parentWidget && aType == eWindowType_popup &&
         IsEffectivelyVisible()) {
+      // put offset into screen coordinates
       nsIntPoint screenPoint = parentWidget->WidgetToScreenOffset();
       viewBounds += nsPoint(NSIntPixelsToAppUnits(screenPoint.x, p2a),
                             NSIntPixelsToAppUnits(screenPoint.y, p2a));
     }
   }
 
+  // Compute widget bounds in device pixels
   nsIntRect newBounds = viewBounds.ToNearestPixels(p2a);
 
+  // Compute where the top-left of the widget ended up relative to the
+  // parent widget, in appunits
   nsPoint roundedOffset(NSIntPixelsToAppUnits(newBounds.x, p2a),
                         NSIntPixelsToAppUnits(newBounds.y, p2a));
-  // mViewToWidgetOffset is added to view coordinates to get widget coordinates
-  mViewToWidgetOffset = roundedOffset - viewBounds.TopLeft();
+
+  // mViewToWidgetOffset is added to coordinates relative to the view origin
+  // to get coordinates relative to the widget.
+  // The view origin, relative to the parent widget, is at
+  // (mPosX,mPosY) - mDimBounds.TopLeft() + viewBounds.TopLeft().
+  // Our widget, relative to the parent widget, is roundedOffset.
+  mViewToWidgetOffset = nsPoint(mPosX, mPosY)
+    - mDimBounds.TopLeft() + viewBounds.TopLeft() - roundedOffset;
 
   return newBounds;
 }

view/src/nsView.h

@@ -178,23 +178,6 @@ public:
 
   virtual ~nsView();
 
-  // This is an app unit offset to add when converting view coordinates to
-  // widget coordinates.  It is the offset in view coordinates from widget
-  // top-left to view top-left.
-  nsPoint ViewToWidgetOffset() const {
-    if (mParent && mParent->GetViewManager() != GetViewManager()) {
-      // The document root view's mViewToWidgetOffset is always (0,0).
-      // If it has a parent view, the parent view must be the inner view
-      // for an nsSubdocumentFrame; its top-left position in appunits
-      // is always positioned at that inner view's top-left, and its
-      // widget top-left is always positioned at that inner view's widget's
-      // top-left, so its ViewToWidgetOffset is actually the same as
-      // its parent's.
-      return mParent->ViewToWidgetOffset();
-    }
-    return mViewToWidgetOffset;
-  }
-
 protected:
   // Do the actual work of ResetWidgetBounds, unconditionally.  Don't
   // call this method if we have no widget.