fix(widgets): keep the section border under a panel scrollbar

[sinelaw]

Fixes sub-issue 3b of #2969 — "Tour explanation box loses its right border at short dock heights".

What was wrong

collect_labeled_section widens a child's scroll region by two columns on purpose, so the bar lands on the section's │ border rather than floating a column inboard (and so a wheel over the border still scrolls the list). render_scrollbar then paints one blank cell per row — correct over a list's own padding, destructive over a border. Every row the bar covered lost the box's right edge.

That is the whole of 3b: the border is not "missing", it is overwritten by a space. Reading the same frame with colours shows it — the last column of each content row is a space carrying the scrollbar's thumb/track background:

lines 1–88                        ^[[38;5;170m^[[48;5;226m ^[[39m   <- thumb, was `│`
                                  ^[[48;5;238m ^[[39m                <- track, was `│`

Which explains the height threshold in the issue (140x45/44/42 fine, 40 and below broken) and why width is irrelevant: a shorter dock is simply what makes the prose overflow its window and earn a scrollbar. Same for "opening the tour into an already-open bottom dock renders borderless immediately" — that dock is shorter from the start.

The fix

render_scrollbar_over_chrome paints the same track/thumb colours but keeps whatever glyph is already in the column, re-styling only its background. Used at the two widget-panel paint sites (split-mounted panels, floating panels), which are exactly the bars whose regions were extended onto the section border. Every other scrollbar in the editor — file browser, settings, keybinding editor, prompt suggestions, buffer scrollbars — is untouched.

The box stays closed, the bar still reads as a bar (border glyph in thumb/track colours), and over an ordinary list row — a space — it paints identically to before, so is_scrollbar_thumb_at / is_scrollbar_track_at and the existing drag/hit-test behaviour are unaffected.

Verified by hand under tmux at 140x36: content rows are back to 137 columns ending in │, with the scrollbar's background still on that column.

Test

test_overflowing_lists_keep_the_section_border in crates/fresh-editor/tests/e2e/code_tour_dock.rs loads the existing overflow-tour fixture (both the Steps rail and the prose overflow), then asserts on rendered output only: on each of the two ╮ columns, the bar is present (a thumb or track cell — this must not pass by having no scrollbar at all) and every content row between the top and bottom borders still carries │.

Without the fix: left: Some(" "), right: Some("│") at the first content row. With it, it passes.

Checks

• cargo fmt, cargo clippy, cargo check --all-targets
• e2e modules most exposed to the change, run before the final commit: code_tour (34), dock (145), orchestrator (145), scrollbar (82) — all pass. The rest is left to CI.

🤖 Generated with Claude Code

https://claude.ai/code/session_013LV5WvsL3drUfQmFRMe4kP

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