ui: improve vertical tab hover expansion and edge polish

[apotenza92]

Summary

Builds on the hover-expanded vertical tabs direction from #1035. This improves collapsed vertical-tab hover expansion/contraction responsiveness and polishes the sidebar/content edge for hover-expanded states.

Motivation

The main UX issue is that hover expansion should feel responsive and forgiving, closer to Arc/Zen. The follow-up visual issue is that the vertical tab sidebar should have a border line drawn consistently for Zen and non-Zen states with 'rounded frame' turned on or off.

Changes

Hover responsiveness

• Makes collapsed vertical tabs expand more consistently on hover with overshoot regions outside of the visible browser window and also allowance for much faster cursor movement to the edge of the window.
• Keeps contraction predictable on mouse exit.
• Makes Zen and non-Zen expansion on hover consistent in animation and speed.

Sidebar edge polish

• Adds a BrowserView-level transparent VerticalTabStripEdgeBorderView overlay strip.
• Lets rounded edge strokes paint outside the sidebar bounds without adding a layout gap.
• Uses GetGenericFrameRadius() for custom sidebar-edge curves.
• Masks overlong underlying content-outline strokes where rounded-frame is enabled.
• Keeps rounded top sidebar/content corner for non-Zen hover-expanded mode even when rounded-frame is disabled.

Verification

Tested locally on macOS.

Related

Related to / builds on #1035. That PR established the hover-expanded vertical-tabs direction and this one adds responsiveness plus visual edge polish.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "ui/views: polish vertical tab sidebar ed..." | Re-trigger Greptile}

[apotenza92]

Pushed cleanup in 3c2cd87: removed dead hover expand timer/callback, coalesced collapse notifications, removed production _for_testing() usage, and cleaned up the leftover OnPaint override. Ready for re-review.

[apotenza92]

Pushed e02e015 to fix the malformed patch hunk and regenerate i18n/source.gen.json. Local lint and i18n generation now pass.

Could someone please approve the fork workflow runs and re-review when you get a chance?

Thanks!

[apotenza92]

Stable Helium is already addressing most of this except the sidebar opening when moving the mouse faster towards it. I will either make a new PR or just an issue addressing that on macOS specifically.