removed some of Axels TODOs and added some notes, avoid clipping operations if nothing needs to be drawn

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@15731 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/servers/app/ViewLayer.cpp

@@ -864,6 +864,7 @@ void
 ViewLayer::Draw(DrawingEngine* drawingEngine, BRegion* effectiveClipping,
 	BRegion* windowContentClipping, bool deep)
 {
+	if (fViewBitmap != NULL || !fViewColor.IsTransparentMagic()) {
 		// we can only draw within our own area
 		BRegion redraw(ScreenClipping(windowContentClipping));
 		// add the current clipping
@@ -875,29 +876,35 @@ ViewLayer::Draw(DrawingEngine* drawingEngine, BRegion* effectiveClipping,
 			BRect rect = fBitmapDestination;
 			ConvertToScreenForDrawing(&rect);
 	
-		// TODO: this messes with the screen clipping, but might not be supposed to do so.
+			// lock the drawing engine for as long as we need the clipping
+			// to be valid
+			if (drawingEngine->Lock()) {
 				drawingEngine->ConstrainClippingRegion(&redraw);
-		// TODO: fDrawState is probably not what we want to use here...
+
+				DrawState defaultDrawState;
 				drawingEngine->DrawBitmap(fViewBitmap, fBitmapSource,
-			rect, fDrawState);
+					rect, &defaultDrawState);
-		drawingEngine->ConstrainClippingRegion(NULL);
+				// NOTE: It is ok not to reset the clipping, that
+				// would only waste time
+				drawingEngine->Unlock();
+			}
 	
 			redraw.Exclude(rect);
 		}
 	
 		if (!fViewColor.IsTransparentMagic()) {
-		// fill visible region with view color
+			// fill visible region with view color,
+			// this version of FillRegion ignores any
+			// clipping, that's why "redraw" needs to
+			// be correct
 			drawingEngine->FillRegion(redraw, fViewColor);
 		}
+	}
 
 	fBackgroundDirty = false;
 
 	// let children draw
 	if (deep) {
-		// before passing the clipping on to children, exclude our
-		// own region from the available clipping
-		effectiveClipping->Exclude(&fScreenClipping);
-
 		for (ViewLayer* child = FirstChild(); child; child = child->NextSibling()) {
 			child->Draw(drawingEngine, effectiveClipping,
 						windowContentClipping, deep);