Is it possible to eliminate flickering entirely when resizing a window?

Use WM_SIZING instead of WM_SIZE and don't forget about WM_ERASEBKGND.

If you go into the System Properties control panel applet, choose the Advanced tab, and then click Settings... in the Performance group box, you'll see a checkbox setting called Show window contents while dragging. If you uncheck that and try resizing a window, you'll see that only the window frame moves until you complete the drag operation, and then the window repaints just once at the new size. This is how window sizing used to work when we had slow, crufty computers.

Now we don't really want to change the setting globally (which you would do by calling SystemParametersInfo with SPI_SETDRAGFULLWINDOWS, but don't really do that because your users won't like it).

What happens when the user grabs the resize border is that the thread enters a modal loop controlled by the window manager. Your window will get WM_ENTERSIZEMOVE as that loop begins and WM_EXITSIZEMOVE when the operation is complete. At some point you'll also get a WM_GETMINMAXINFO, which probably isn't relevant to what you need to do. You'll also get WM_SIZING, WM_SIZE messages rapidly as the user drags the sizing frame (and the rapid WM_SIZEs often lead to WM_PAINTs).

The global Show window contents while dragging setting is responsible for getting the rapid WM_SIZE messages. If that setting is off, you'll just get one WM_SIZE message when it's all over.

If your window is complicated, you probably have layout code computing stuff (and maybe moving child windows) in the WM_SIZE handler and a lot of painting code in the WM_PAINT handler. If all that code is too slow (as your sample 320 ms delay suggests), then you'll have a flickery, jerky experience.

We really don't want to change the global setting, but it does inspire a solution to your problem:

Do simpler drawing during the resize operation and then do your (slower) complex drawing just once when the operation is over.

Solution:

1. Set a flag when you see the WM_ENTERSIZEMOVE.
2. Change your WM_SIZE handler to check the flag and do nothing if it's set.
3. Change your WM_PAINT handler to check the flag and do a simple, fast fill of the window in a solid color if it's set.
4. Clear the flag when you see WM_EXITSIZEMOVE, and then trigger your layout code and invalidate your window so that everything gets updated based on the final size.

If your slow window is a child rather than your application's top-level window, you'll have to signal the child window when the top-level window gets the WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE in order to implement steps 1 and 4.

When the window is updated during a drag operation, then the OS has to show something in the extended window region. If you can't provide anything then it will show the background until you do. Since you didn't specify any background you get blackness. Surely you ought to be specifying a background brush? Simply adding the following to your code makes the behaviour more palatable:

wcex.hbrBackground = GetStockBrush(DKGRAY_BRUSH);

However, if you take as long as 320ms to respond to a WM_PAINT then you ruin the resize UI for the user. It becomes jerky and unresponsive. The system is designed around the assumption that you can paint the window quickly enough for dragging to feel smooth. The right way to fix your problem is to make WM_PAINT run in a reasonable time.

If you really can't achieve quick enough painting for smooth dragging then I suggest a couple of alternatives:

1. Disable window updates during dragging. I'm sure this can be done for individual windows, but I can't remember how to do it off the top of my head.
2. Paint something fake whilst a resize/drag is active, and postpone the real painting until when the resize/drag has completed. Listening for WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE are the keys to this. This Microsoft sample program illustrates how to do that: https://github.com/microsoft/Windows-classic-samples/blob/master/Samples/Win7Samples/winui/fulldrag/