There are cases where the desktop embeddings needed to instruct the engine to redraw (e.g., after certain window manager operations on Linux that discard the previous backing buffer). Currently there's no way to do that explicitly, so we've just send a window metrics update with the current metrics. This doesn't capture what is conceptually being requested, and there's an obvious potential for unintended side-effects of reasonable-seeming changes in the engine (such as optimizing away metrics updates that have the current values).