feat(tabs): split-tab context menu, and stop an open menu swallowing the next right-click

[kipavy]

What

Right-clicking a unified split tab did nothing: only the session-tab button carried an onContextMenu, so a session merged into a split lost the menu its own tab had. Reported as "the tab context menu doesn't appear on local shells" — local shells were a red herring, a split was the common factor.

The new menu is tab-scoped; anything acting on a single session stays on the pane header, where the target is unambiguous:

• Broadcast input to all panes — toggles the tab's own broadcastActive, until now reachable only from the pane-header button
• Panes › — submenu listing the tab's panes, numbered (1 · bash) because a duplicate-into-split yields two identical names; picking one focuses it (and un-maximizes a maximized sibling)
• Split apart into tabs — every pane becomes its own tab; previously only possible by dragging each pane onto the titlebar
• Close tab (N sessions) — the count warns that it takes down more than one session

Chasing that bug surfaced a second one, fixed in the follow-up commit: the menu sat above a full-screen backdrop that closed it on mousedown, and that backdrop also caught the right-click meant for whatever was under it. With one menu open, right-clicking another target did nothing at all and the stale menu stayed — the likely source of the "sometimes there's no menu" reports. The backdrop is gone; the menu now closes from window-level mousedown/contextmenu capture, ignoring presses inside itself or inside a [data-menu-portal] submenu. Capture runs before React's handlers and does not preventDefault, so the event reaches the element under the pointer and it opens its own menu in the same event.

Every layout mutation targets the active split tab, so each entry activates its tab first.

Verification

Driven in the headless dev build, real pointer input:

• each of the four entries does what it says — broadcast label flips, pane focus moves the active-pane border, split apart dissolves the tab into plain tabs, close removes the tab and both sessions
• with a menu open, right-clicking another tab now retargets the menu; left-click outside closes; the Panes › submenu still opens on hover and its entry still fires

tsc --noEmit clean. Full suite 521/522 files, 3991/3992 tests — the one failure, teamService.transport.test.ts > missing jwt triggers refresh, is a contention flake that passes 10/10 alone and touches nothing here.

Caveat on the tests

ContextMenu.test.tsx does not prove the swallowed-right-click bug: jsdom's fireEvent dispatches straight to the node, so a covering backdrop is invisible to it and the old code passes the retarget assertion. The test covers the close semantics only, and says so in a comment; the hit-testing half is proven by the live run.

Behavior change worth knowing

Without the backdrop, a left-click outside a menu now also reaches what it lands on, for every ContextMenu caller (hosts, snippets, keychain, file pane, pane header).