Add macOS Vibrance effect behind the File Browser sidebar

[faisal]

Give the file browser the translucent "Sidebar" NSVisualEffectView material introduced in macOS 10.10, matching how sidebars look elsewhere in the system.

The outline view's background was previously an opaque, dynamic NSColor.controlBackgroundColor (added to fix a rendering bug before any vibrancy view existed). Make it transparent again so the vibrancy view underneath supplies the adaptive light/dark background instead.

[dayglojesus]

@faisal Thanks for the PR. I can see why some folks would want this effect, but personally I find Liquid Glass styling distracting. I do not want to know what is behind a window, and I do not want ghostly files and folders rising up into the file browser frame.

I built and ran it on macOS 27.0 and it renders correctly in both appearances, so this is a design objection, not a technical one. I think it would be more appropriately implemented as a toggle in Preferences, default off. The change reduces to the three settings in FileBrowserView's init (outline background, scroll view background, and the effect view underneath), so a toggle is one branch there plus a checkbox and a notification to flip live views. If you would rather keep it small, a defaults key read at init and documented with the other hidden keys in the Help page would also do.

Happy to review it again in that form.

[faisal]

@faisal Thanks for the PR. I can see why some folks would want this effect, but personally I find Liquid Glass styling distracting. I do not want to know what is behind a window, and I do not want ghostly files and folders rising up into the file browser frame.

I built and ran it on macOS 27.0 and it renders correctly in both appearances, so this is a design objection, not a technical one. I think it would be more appropriately implemented as a toggle in Preferences, default off. The change reduces to the three settings in FileBrowserView's init (outline background, scroll view background, and the effect view underneath), so a toggle is one branch there plus a checkbox and a notification to flip live views. If you would rather keep it small, a defaults key read at init and documented with the other hidden keys in the Help page would also do.

Happy to review it again in that form.

How would you feel about breaking this into two PRs: this one to add a defaults key and a second one for UI?

[dayglojesus]

How would you feel about breaking this into two PRs: this one to add a defaults key and a second one for UI?

It would be much easier to test the features in unison.