Main reason for doing this is that the navigation buttons are very
close to the file list now, making them much faster to reach.
Initially we let the upper bar (the one with the file path, navigation
and display buttons) use full area width, because designs back then had
more horizontal space problems. The designs have changed meanwhile, and
horizontal space is less of an issue.
However, when the file browser is shrunk horizontally, or if it's open
in a small area (e.g. see "Shading" workspace), having the tool region
open brings back the space issues. But even the file list layout becomes
problematic then, and the same issue was present before the new file
browser design, so we've decided this is an acceptable tradeoff.
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