On the newest version of Git available via Homebrew (2.19.1), when the gui tool needs to repaint the screen (refresh a file, change a focused file, stage files) the window that is repainted flashes a dark gray before returning to white. This was not occurring on High Sierra yesterday, but has occurred today. I believe it was occurring on an older version of Git as well, but I updated to ensure that it wasn't something that was already solved (it was not). I can provide more diagnostic data if needed. Thanks! Matt