Does SetWindowPos paint immediately

Question

On every mouse move event, I am moving my window. From the documentation it appears that if the move is successful SetWindowPos return a non-zero value.

I wanted to confirm my inference that every time SetWindowPos returns a non-zero value, the move has already happened. Which means, the window has actually been re-painted at the new position and only then does the function return. It is NOT the case that move will happen a little while later (when some kind of message is processed) after the function has returned.

Answer 1

I wanted to confirm my inference that every time SetWindowPos returns a non-zero value, the move has already happened. Which means, the window has actually been re-painted

This is false assumption. Successful move and non-zero return has nothing to do with painting.

The API changes position and non-zero returned value confirm that new values were accepted. The API does not promise, does not do the complete repaint cycle as a part of its execution. More to that, change in position, Z-order etc. is likely to affect visibility of other windows too, which in turn need repaint, which in turn has to happen on respective threads. The repaints are scheduled, they are not synchronized with SetWindowPos return. Documentation on MSDN does not have any promises regarding repaints/updates. Then zero return from the API is an indication that your arguments are not accepted at all (esp. invalid window handle).

Bonus reading :

When you call the SetWindowPos function, the window manager updates the window size, position, whatever, and then it goes around repainting the windows that were affected by the operation. By default, the SetWindowPos function does a quick-repaint of the windows before returning. After the function returns, the normal WM_PAINT message does the real work of painting the window. The quick-repaint is done so that there is immediate feedback that the window did change its size, position, whatever.

Answer 2

SetWindowPos returns imidialty after it has done ist job.

But the main painting is defered until WM_NCPAINT and WN_PAINT are received by the window. From my expirience I can say that changes to the frame are often drawn directly by SetWindowPos.

If you want the window completely redrawn call UpdateWindow or RedrawWindow (with appropriate flags) after the call to SetWindowPos