TUI ext panel: only top section header toggles on click in git_insights (2 of 3 still don't)

[JDonaghy]

Symptom

After PR for #483/#484/#485 (commit `fdd83ca`), single-click on section headers in plugin-provided extension panels toggles only the top section. Other sections (e.g. Log and Stash in `git_insights`) still don't visibly toggle on click.

Repro

1. Open the `git_insights` extension panel (3 sections: Branches / Log / Stash).
2. Click the Branches header → toggles correctly.
3. Click the Log header → no visible toggle.
4. Click the Stash header → no visible toggle.

`Tab` on a selected section header still toggles correctly for all three.

What was attempted

The original #484 fix at `src/tui_main/mouse.rs:2299` gated `handle_ext_panel_key("Return", ...)` on `!is_double` to prevent terminal-emitted double-Down events from un-toggling. That fix made one section work but the other two remained broken — so the double-fire theory wasn't the (whole) root cause.

Cause (unknown)

Code review of `ext_panel_flat_to_section`, `ext_panel_flat_len`, and the `Return` handler in `ext_panel.rs:308+` all check out for sections 0/1/2 on paper. The flat-idx math is correct whether sections are expanded or collapsed.

Possible angles for the next agent:

• Instrument the click handler at mouse.rs:2289+ — log `flat_idx`, the result of `flat_to_section`, and `ext_panel_sections_expanded` before/after. Compare working (Branches) vs non-working (Log/Stash) cases.
• Check whether the plugin's `set_items` callbacks are re-emitting items in response to a side effect of the click and inadvertently resetting `ext_panel_sections_expanded`.
• Compare against keyboard `Tab` path — both route through the same engine method, but the click path adds the selection-then-Return sequence. Maybe selection update via the click path produces a different state.

Files

• `src/tui_main/mouse.rs:2289-2306` — click dispatch
• `src/core/engine/ext_panel.rs:308-388` — `Return` handler for section headers / expandable items
• `src/core/engine/ext_panel.rs:98-121` — `flat_to_section`

Notes

Surfaced during smoke testing of the #483/#484/#485 fix branch (`fdd83ca`).