As described by @Catrope:

Before this patch in T216131, the title bar of the help panel would briefly get the pending animation (cross-hatching) when you focus the search input. But with this patch, this happens on every key press. More specifically, on every key press, first the title bar will become pending, then it'll immediately stop being pending again, and only then will the search input itself become pending (and stay that way until the results arrive). Probably not a big deal but it looks a bit weird.

I traced the cause in the debugger, and it seems to come from calling .executeAction( 'search' ): any call to executeAction sets the pending state on the title bar.