fix: improve table of contents scrolling

[hannesrudolph]

What Problem This Solves

Long “On this page” outlines stopped above the bottom of the window, and their scrollbar started above the heading with a distracting native track.

User Impact

The outline now uses the available window height, with a thin thumb on hover or keyboard focus, a transparent track, and fades only where more items remain. The scrollbar begins level with the heading, whose position is preserved. Keyboard focus and the automatically highlighted entry stay clear of Ask Molty.

Why This Change Was Made

The heading inset now sits outside the scrolling area. Scroll-end space keeps the last link reachable above the chat control, and automatic scrolling uses the same visible bounds as keyboard focus. The overflow cues follow the existing left navigation.

Evidence

• 274 tests passed; one existing test skipped. All 6 focused anchor tests passed after the scrollspy correction.
• Focused Chromium and WebKit checks passed for the scrollbar, top/bottom overflow states, keyboard reachability, automatic active-link scrolling, changing outline length, page navigation, and compact-layout handoff.
• Source review completed with no actionable findings.
• Dark/light and Persian RTL previews verified. At 1440×900, the heading remains at y=107; the scrolling area now runs from y=107 to y=900 rather than y=64 to y=820.

Persian RTL preview