Upstream vscode UI redesign #3038

Upstream: microsoft/vscode#334285.

There is a UI redesign that's probably going to land soon in vscodium from upstream vscode which has a bunch of usability issues. For now, there seems to be an option to disable it via workbench.experimental.modernUI (and related settings keys to tweak the appearance).

Not sure how hard it'll be to maintain the old UI once the experimental toggle is removed. A potential patch here?

Note: Shouldn't have to be said, but just in case, please don't spam the upstream issue.