Problem:
Flickering may occur when paging up/down in big files, as ranges for semantic
tokens are requested. This happens with LSP servers like gopls which return
"/full" semantic tokens if the file is too big, where we fall back to
viewport-range token retrievals.
Solution:
Broaden the requested ranges to one viewport of "overscan" on each side plus
some padding if possible:
(viewport_topline - viewport_height)..(viewport_botline + viewport_height)
(cherry picked from commit 3ed78daf83)
fded370b3e