Bug 382595: Lines across embedded svg when scrolling. Followup rounding fix. r+sr=roc

gfx/public/nsRect.h

@@ -165,9 +165,9 @@ struct NS_GFX nsRect {
   // Scale by aScale, converting coordinates to integers so that the result
   // is the larges integer-coordinate rectangle contained in the unrounded result
   nsRect& ScaleRoundIn(float aScale);
-  // Scale by aScale, converting coordinates to integers so that the result
-  // contains the same pixel centers as the unrounded result
-  nsRect& ScaleRoundPreservingCenters(float aScale);
+  // Scale by the inverse of aScale, converting coordinates to integers so that
+  // the result contains the same pixel centers as the unrounded result
+  nsRect& ScaleRoundPreservingCentersInverse(float aScale);
 
   // Helpers for accessing the vertices
   nsPoint TopLeft() const { return nsPoint(x, y); }

gfx/src/nsRect.cpp

@@ -204,12 +204,12 @@ nsRect& nsRect::ScaleRoundIn(float aScale)
   return *this;
 }
 
-nsRect& nsRect::ScaleRoundPreservingCenters(float aScale)
+nsRect& nsRect::ScaleRoundPreservingCentersInverse(float aScale)
 {
-  nscoord right = NSToCoordRound(float(XMost()) * aScale);
-  nscoord bottom = NSToCoordRound(float(YMost()) * aScale);
-  x = NSToCoordRound(float(x) * aScale);
-  y = NSToCoordRound(float(y) * aScale);
+  nscoord right = NSToCoordRound(float(XMost()) / aScale);
+  nscoord bottom = NSToCoordRound(float(YMost()) / aScale);
+  x = NSToCoordRound(float(x) / aScale);
+  y = NSToCoordRound(float(y) / aScale);
   width = (right - x);
   height = (bottom - y);
   return *this;

view/src/nsView.cpp

@@ -358,7 +358,7 @@ nsRect nsView::CalcWidgetBounds(nsWindowType aType)
   }
 
   nsRect newBounds(viewBounds);
-  newBounds.ScaleRoundPreservingCenters(1.0f / p2a);
+  newBounds.ScaleRoundPreservingCentersInverse(p2a);
 
   nsPoint roundedOffset(NSIntPixelsToAppUnits(newBounds.x, p2a),
                         NSIntPixelsToAppUnits(newBounds.y, p2a));