Fix bug 16200 (mostly) for both GFX and native scrollbars by rounding scroll requests to the nearest pixel and, from then on, only using that rounded value. Previously, disagreements (of a fraction of a pixel) between what was on the screen and what was supposed to be there caused rounding problems (particularly at less common logical resolutions). There is a chance this may not be necessary in the future, because transform changes may fix this.

r=dcone

view/src/nsScrollPortView.cpp

@@ -232,21 +232,6 @@ NS_IMETHODIMP nsScrollPortView::ScrollTo(nscoord aX, nscoord aY, PRUint32 aUpdat
     nsIDeviceContext  *dev;
 	float             t2p;
 	float             p2t;
- 
-	// notify the listeners.
-	PRUint32 listenerCount;
-        const nsIID& kScrollPositionListenerIID = NS_GET_IID(nsIScrollPositionListener);
-	nsIScrollPositionListener* listener;
-	if (nsnull != mListeners) {
-		if (NS_SUCCEEDED(mListeners->Count(&listenerCount))) {
-			for (PRUint32 i = 0; i < listenerCount; i++) {
-				if (NS_SUCCEEDED(mListeners->QueryElementAt(i, kScrollPositionListenerIID, (void**)&listener))) {
-					listener->ScrollPositionWillChange(this, aX, aY);
-					NS_RELEASE(listener);
-				}
-			}
-		}
-	}
 
     mViewManager->GetDeviceContext(dev);
 	dev->GetAppUnitsToDevUnits(t2p); 
@@ -282,10 +267,34 @@ NS_IMETHODIMP nsScrollPortView::ScrollTo(nscoord aX, nscoord aY, PRUint32 aUpdat
     nscoord aXpx = NSTwipsToIntPixels(aX, t2p);
     nscoord aYpx = NSTwipsToIntPixels(aY, t2p);
 
+    aX = NSIntPixelsToTwips(aXpx,p2t);
+    aY = NSIntPixelsToTwips(aYpx,p2t);
+
+    // do nothing if the we aren't scrolling.
+    // this needs to be rechecked because of the clamping and
+    // rounding
+    if (aX == mOffsetX && aY == mOffsetY)
+        return NS_OK;
+
     // figure out the diff by comparing old pos to new
     dxPx = mOffsetXpx - aXpx;
     dyPx = mOffsetYpx - aYpx;
-    
+ 
+	// notify the listeners.
+	PRUint32 listenerCount;
+        const nsIID& kScrollPositionListenerIID = NS_GET_IID(nsIScrollPositionListener);
+	nsIScrollPositionListener* listener;
+	if (nsnull != mListeners) {
+		if (NS_SUCCEEDED(mListeners->Count(&listenerCount))) {
+			for (PRUint32 i = 0; i < listenerCount; i++) {
+				if (NS_SUCCEEDED(mListeners->QueryElementAt(i, kScrollPositionListenerIID, (void**)&listener))) {
+					listener->ScrollPositionWillChange(this, aX, aY);
+					NS_RELEASE(listener);
+				}
+			}
+		}
+	}
+
 	if (nsnull != scrolledView)
 	{
         // move the scrolled view to the new location

view/src/nsScrollingView.cpp

@@ -1266,27 +1266,41 @@ NS_IMETHODIMP nsScrollingView::ScrollTo(nscoord aX, nscoord aY, PRUint32 aUpdate
 
 	mClipView->GetDimensions(&clipSize.width, &clipSize.height);
 
+	// Clamp aX
+
+	if ((aX + clipSize.width) > mSizeX)
+		aX = mSizeX - clipSize.width;
+
+	if (aX < 0)
+		aX = 0;
+
+	// Clamp aY
+
+	if ((aY + clipSize.height) > mSizeY)
+		aY = mSizeY - clipSize.height;
+
+	if (aY < 0)
+		aY = 0;
+
+	aX = NSIntPixelsToTwips(NSTwipsToIntPixels(aX, t2p), p2t);
+	aY = NSIntPixelsToTwips(NSTwipsToIntPixels(aY, t2p), p2t);
+
+	// do nothing if the we aren't scrolling.
+	if (aX == mOffsetX && aY == mOffsetY)
+		return NS_OK;
+
 	mVScrollBarView->GetWidget(widget);
 
 	if (nsnull != widget) {
 		nsIScrollbar* scrollv = nsnull;
 		if (NS_OK == widget->QueryInterface(NS_GET_IID(nsIScrollbar), (void **)&scrollv)) {
-			// Clamp aY
-
-			if ((aY + clipSize.height) > mSizeY)
-				aY = mSizeY - clipSize.height;
-
-			if (aY < 0)
-				aY = 0;
-
 			// Move the scrollbar's thumb
 
 			PRUint32  oldpos = mOffsetY;
-			PRUint32  newpos = NSIntPixelsToTwips(NSTwipsToIntPixels(aY, t2p), p2t);
 
-			scrollv->SetPosition(newpos);
+			scrollv->SetPosition(aY);
 
-			dy = NSTwipsToIntPixels((oldpos - newpos), t2p);
+			dy = NSTwipsToIntPixels((oldpos - aY), t2p);
 
 			NS_RELEASE(scrollv);
 		}
@@ -1299,22 +1313,13 @@ NS_IMETHODIMP nsScrollingView::ScrollTo(nscoord aX, nscoord aY, PRUint32 aUpdate
 	if (nsnull != widget) {
 		nsIScrollbar* scrollh = nsnull;
 		if (NS_OK == widget->QueryInterface(NS_GET_IID(nsIScrollbar), (void **)&scrollh)) {
-			// Clamp aX
-
-			if ((aX + clipSize.width) > mSizeX)
-				aX = mSizeX - clipSize.width;
-
-			if (aX < 0)
-				aX = 0;
-
 			// Move the scrollbar's thumb
 
 			PRUint32  oldpos = mOffsetX;
-			PRUint32  newpos = NSIntPixelsToTwips(NSTwipsToIntPixels(aX, t2p), p2t);
 
-			scrollh->SetPosition(newpos);
+			scrollh->SetPosition(aX);
 
-			dx = NSTwipsToIntPixels((oldpos - newpos), t2p);
+			dx = NSTwipsToIntPixels((oldpos - aX), t2p);
 
 			NS_RELEASE(scrollh);
 		}