I'll explain it: In the olden Windows days, the application is in charge of painting its titlebar by handling the WM_NCPAINT message. The default window handler

I'll explain it: In the olden Windows days, the application is in charge of painting its titlebar by handling the WM_NCPAINT message. The default window handler for this will paint the Windows Vista-era buttons. During Windows 7's development, the responsibility of painting window frames moved from the application to a separate process known as the Desktop Window Manager, "dwm.exe". DWM tells the app that it should stop drawing its own frame when the application is windowed, and tells it is in charge of drawing "the frame" when fullscreen. This is communicated using an asynchronous protocol (private window messages), thus during the transition it's possible for the window's idea of its frame state to be "out of sync" with the real state because the client hasn't processed all of its messages yet.