Scroll redraw on Windows lead to drawing artifacts

The reworked scrolling of the FigureCanvas didn't correctly calculated
the exposure areas which need to be redrawn after scrolling. This led to
drawing artifacts.

org.eclipse.draw2d/src/org/eclipse/draw2d/FigureCanvas.java

@@ -411,24 +411,20 @@ private void performScroll(int hOffset, int vOffset) {
 		int dy = -vOffset + getViewport().getViewLocation().y;
 		Rectangle clientArea = getViewport().getBounds().getShrinked(getViewport().getInsets());
 		Rectangle blit = clientArea.getResized(-Math.abs(dx), -Math.abs(dy));
-		Rectangle expose = clientArea.getCopy();
 		Point dest = clientArea.getTopLeft();
 
-		expose.width = Math.abs(dx);
+		blit.width = Math.max(blit.width, 0);
+		blit.height = Math.max(blit.height, 0);
+
 		if (dx < 0) { // Moving left?
 			blit.translate(-dx, 0); // Move blit area to the right
-			expose.x = dest.x + blit.width;
-		} else {
-			// Moving right
+		} else { // Moving right
 			dest.x += dx; // Move expose area to the right
 		}
 
-		expose.height = Math.abs(dy);
 		if (dy < 0) { // Moving up?
 			blit.translate(0, -dy); // Move blit area down
-			expose.y = dest.y + blit.height; // Move expose area down
-		} else {
-			// Moving down
+		} else { // Moving down
 			dest.y += dy;
 		}
 
@@ -438,7 +434,7 @@ private void performScroll(int hOffset, int vOffset) {
 		getViewport().setHorizontalLocation(hOffset);
 		getViewport().setVerticalLocation(vOffset);
 		getViewport().setIgnoreScroll(false);
-		redraw(expose.x, expose.y, expose.width, expose.height, true);
+		scrollRedraw(dx, dy, clientArea, blit);
 	}
 
 	private void scrollChildren(int dx, int dy, Rectangle blit, Point dest) {
@@ -462,6 +458,26 @@ private void scrollChildren(int dx, int dy, Rectangle blit, Point dest) {
 		}
 	}
 
+	private void scrollRedraw(int dx, int dy, Rectangle clientArea, Rectangle blit) {
+		Rectangle expose = clientArea.getCopy();
+		if (dx != 0) {
+			expose.width = Math.abs(dx);
+			if (dx < 0) { // Moving left?
+				expose.x += blit.width; // Move expose area right
+			}
+			redraw(expose.x, expose.y, expose.width, expose.height, true);
+		}
+		if (dy != 0) {
+			expose.x = clientArea.x;
+			expose.width = clientArea.width;
+			expose.height = Math.abs(dy);
+			if (dy < 0) { // Moving up?
+				expose.y += blit.height; // Move expose area down
+			}
+			redraw(expose.x, expose.y, expose.width, expose.height, true);
+		}
+	}
+
 	/**
 	 * Sets the given border on the LightweightSystem's root figure.
 	 *