content (or background) remains when draggin the splitter bar to resize two panels

Question

I have a main window with 3 child: hwndTocBox (left panel), hwndSplitter and hwndCanvas (right panel). hwndTocBox has a child hwndTreeView, which is a TreeView control. When I drag hwndSplitter to the right (i.e want to make hwndTocBox and hence hwndTreeView bigger), the content (and background?) of hwndCanvas and hwndSplitter will remain for a while. (When I drag the splitter to the left, there is no problem at all.)

When hwndSplitter is draged, it uses DeferWindowPos() to resize and move hwndTocBox, hwndSplitter and hwndCanvas. When hwndTocBox is resized, in WM_SIZE case of its windows procedure, it resizes hwndTreeView (still using DeferWindowPos(), since it resizes not only hwndTreeView but also others).

I have tried to use CLIPCHILDREN and WS_CLIPSIBLINGS in several places, but it doesn't solve the problem.

Why the contents remain there for a while and erased later?

Could you indicate me how to solve this issue, please.

Answer 1

You'll need to repaint the portions of the window that you've resized, otherwise you'll get these strange artifacts. There's a reason they look like smears, because that's almost exactly what they are. A certain portion of the content was painted and appeared on screen normally. Then you resized the window. The portion that had already been painted was not repainted because you didn't invalidate it, but the newly-exposed portion had to be repainted because there was nothing there before.

The fix is simple: add a call to the InvalidateRect() function at the bottom of your resizing code to ensure that the portion of the window you're resizing gets redrawn the next time that the window processes a WM_PAINT message.

If you want to make sure that a WM_PAINT message gets processed immediately (resulting in the immediate redrawing of your window's affected regions), follow up with a call to the UpdateWindow() function. But this really shouldn't be necessary. It's much more efficient to postpone all of the redrawing to later when everything is finalized, rather than doing it incrementally. Relatively speaking, repainting a window is an expensive operation.