handle multiple windows for same buffer

[FelipeLema]

I cannot reproduce this consistently, but moving around splits with more-than-one buffers (2 should be enough?), using at least 2 splits for 1 buffer, makes the semshi highlighting go away. If I jump back into this window, semshi will re-highlight it all right

it seems that jumping around makes "the latest window focused" not being considered? I'm not sure if semshi will clear previous highlights from "anything that's not the current window"

[FelipeLema]

I can do a PR to iterate over all windows / viewports for a buffer, but that will have a slight impact on performance

[wookayin]

I've seen this bug before and it was quite hard to reproduce for me too. But if you have any clues, I'd be happy to investigate together.