This problem appeared when upgraded to E 0.19.0 and persists since. Oh yes I've got a ton of tabs open but most of them are suspended using The Great Suspender.
So, at random times maybe once an hour a yellow tooltip - either on the web page, on the tab bar or over the add-ons/extensions icons - appears all white and persists there on the screen, even if I hide or move the chromium window and all. At this time browser is still usable but only until the next such tooltip is invoked. It replaces the previous one, persists on the screen and freezes chromium completely.
When running from terminal, no messages appear there when the whole thing happens. Nothing more I can tell. Using OpenGL rendering and some basic AMD graphics card.