c++ - Flickering on window when resizing from left side

Question

It seems that my window is flickering whenever I move and window and resize at the same time. This usually occurs when sizing is done from the left side of the window.

Why does this flickering happen? In other words, what is the OS doing when you reposition a window?

Note: I do not experience flickering when resizing from the right side which means the window is not necessarily moving its origin X and Y.

Answer 1

Resizing a window under Windows involves several messages sent between the OS and the window's handler (the lpfnWndProc member of WNDCLASSEX structure used to register the window's class). You can discover them by yourself using some message monitoring tool. Spy++ that comes with Visual Studio is one such tool.

One interesting message is WM_NCCALCSIZE: this message, called during window resizing, can generate two rectangles (when WVR_VALIDRECTS flag is set): source and target specifying what content of the old window's client area can be "reused" at the new window's position. By default it's assumed that the top-left corner is a pivot:

• resizing the left or top border causes the old window's content to get copied to preserve the pivot;
• resizing the right ot bottom border copies nothing because the top-left corner of the window did not move.

This default copying can cause flicker, if it does not correspond to the way you position visuals during repaint. For example, everything that is displayed relative to the right or bottom border will be misplaced after resize from the left or top border: these objects will get moved unnecessarily leaving strange mix of old and new things after such resize, because only non-copied pixels will be repainted. If you try to cure the mess with InvalidateRect during, say, WM_SIZE you will get the flicker (the time interval where things are misplaced is very short but it still exists).

The easiest way to disable this behavior is by setting the CS_HREDRAW and CS_VREDRAW Class Styles for your window.