I know that WM_PAINT tells a window that it needs to repaint itself entirely, but apparently that's not the message that gets sent when it's been covered partially and then the window that was in front of it is no longer in the way and it needs to repaint the dirty portion. Does anyone know what message is sent in this case?
EDIT: Found the problem:
The issue involved a Delphi control I wrote to embed a SDL rendering surface on a Delphi form. SDL has an API to build its renderer on another window's HWND, and it marks it as a "foreign window".
SDL usually handles WM_PAINT internally, so I ended up having to add some code to SDL's WindowProc to forward the message on to the external WindowProc if it's a foreign window. That was working sometimes, but it turns out there was a glitch that was stripping the foreign window flag from the window's data structure, so it was swallowing the message instead of passing it on to my app. Once I fixed that, I started getting the WM_PAINT messages all the time.
Why do you say it's apparently not? WM_PAINT should be called for partial redraws (the updated rect is returned by BeginPaint or GetUpdateRect). If it doesn't appear to be getting called, there may be a bug elsewhere in your app that's preventing it. What are you seeing that leads you to believe that it's not working?