The salvaged fix reserved a fixed 48px drag handle whenever a top-edge tab
strip is visible. That is right when the tabs share the titlebar band (every
tab is `no-drag` and the list scrolls, so a crowded strip covers all the
draggable pixels — #112964), but it also shrank the handle in the layout
6cc51b407c introduced for narrow zones: tabs drop to a second row below the
window controls and the whole titlebar row above them is free. There the
flexible handle must keep spanning the row, or a narrow window loses most of
its drag area.
Scope the fixed width to `headerVisible && tabsInTitlebar`; otherwise keep
the flex-1 handle exactly as origin/main had it.
Tests: replace the contributor's single assertion with two invariants in
tree-group.test.tsx — a crowded titlebar strip renders a fixed-width handle
outside the `role="tablist"` scroller (red on origin/main), and the
below-controls layout keeps a flexible, width-less handle (red on the PR as
submitted). The `TITLEBAR_DRAG_HANDLE_WIDTH === 48` pin in titlebar.test.ts
is dropped: it is a change-detector on a constant, not a behaviour.
1aa296fbcd