This has been discussed internally, but chromium is partly broken these days.
Most specifically, windows refresh does strange things under some circumstances. The circumstances are well-known (thanks to matthieu@): "modern" systems use some composition manager for eye-candy on their display. So if you're using a shiny window manager, you won't see an issue. Old-style window managers, such as fvwm, fvwm2 (from ports) and cwm don't. Hence the breakage. Work-around: start a composition manager, such as xcompmgr from base xenocara. Cry since you lost your background image or moire pattern (fvwm-root, from ports, does know about composition managers). We're currently in the process of reporting the problem upstream. Outside of OpenBSD, most people don't use primitive window managers, so they don't see the issue. It probably started around when chromium switched to Aura for its gfx system...
