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)
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