Description Peter Severin 2012-11-08 08:02:16 EST

Created attachment 223348 [details]
SWT test case that shows the issue.

I am trying to implement drag & drop of resource files from Project Explorer into a GEF editor. When the file is dragged over I want to show some feedback under the mouse cursor. This results in a bad repaint issue on Windows only. Specifically the area under the dragged image is not repainted correctly.

After looking into it I narrowed it down to SWT. GEF uses a specific way of repainting the canvas, specifically it calls Control#redraw() that is immediately followed by Control#update(). This call to update is the one that causes the issue.

After looking more into it I narrowed my search to the following code in DropTarget class:

DropTarget#DragOver {
...
	notifyListeners(event.type, event);
	refresh();
...
}

Refresh method is the one that normally forces the repaint:

void refresh() {
	if (control == null || control.isDisposed()) return;
	int /*long*/ handle = control.handle;
	RECT lpRect = new RECT();
	if (OS.GetUpdateRect(handle, lpRect, false)) {
		OS.ImageList_DragShowNolock(false);
		OS.RedrawWindow(handle, lpRect, 0, OS.RDW_UPDATENOW | OS.RDW_INVALIDATE);
		OS.ImageList_DragShowNolock(true);
	}
}

However if Control#update is called by one of the drop listeners then refresh() method does nothing. This means that the image under the cursor is not updated properly as OS.ImageList methods are never called.

I created a pure SWT test case that reproduces this issue.

Bug 218823 looks similar to this but I am not sure this is the same problem.

Bug 393514 in GEF is the one blocked by this issue.