I have been using vscodes default high contrast theme for years; somewhere in the past few releases a dotted box has been introduced, but I can't find anything in the release notes about it and inspecting tokens doesn't help. Can't narrow it down with developer window either.
Is there a way to disable / alter the white dotted border:
It appears to be reproducible in these scenarios:
- Place cursor anywhere in the word boundary and navigate to any other panel/ui element that's not the editor (using shortcuts or mouse doesn't matter)
- Pressing Ctrl+F anywhere on the word
I am currently using vscode 1.75

Same issue: find match highlight border color - seems to be a reversion see
Borders styles for selection and word highlight:
But a PR was submitted and merged (https://github.com/microsoft/vscode/pull/46956) to change the dotted high contrast theme
find...border colors to solid. So you should upvote the first issue mentioned above.A workaround has been provided until a css rule has been corrected, see
from find match highlight border color.