File status bar hides when scrolling down, forcing users to scroll up to mark file as 'viewed'

[gosr]

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Bug

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This has happened recently.
When you're in review mode of a PR, and it contains many files, often you need to scroll down to see more content. The bar with the file name and the "Viewed" checkbox gets hidden. So now you have to scroll up again to mark the file as viewed. This is super annoying.
Earlier, the file bar was sticky.

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[annervisser]

This seems to be caused by incorrect use of a sass variable as value of a css variable:

In scss you need to escape the value like so (see also https://sass-lang.com/documentation/breaking-changes/css-vars/):

--pr-toolbar-sticky-header-height: #{$sticky-header-height};