The GHPR browser extension improves the UI of GitHub PRs. Install from here.

• Adapts the PR sidebar to the lengths of the file names;
• Lets you resize the sidebar yourself;
• Dims and folds reviewed files and folders in the pull request tree;
• Highlights in the tree the visible files you're reviewing on the right pane.

Pin the extension and select the options that suit you!

Strike through reviewed files. Fold reviewed folders!

Visible files in the right pane are highlighted in the tree.

Sidebar can be resized (manually or automatically).