feat: the pop-out title bar, done well, and a sidebar that answers sooner

[Maxaubert]

Follow-up to #189 (#162), from the owner on 2026-10-11: "a pop out title bar is best actually we just need to do it well. same for the sidebar ... it reacts a bit too late ... could be better if it was like 20px further into the window, and the hidden titlebar should be like zen browser's title bar ... when the sidebar is open the titlebar drops down but only from the sidebar edge and all the way to the right ... also the tabs should start a bit further down on the sidebar so that you can grab that area. also make sure that the titlebar area is usable for content".

• The pop-out title bar, done well. With vertical tabs and Show title bar off, the terminal has the whole height. A bar drops over it within 16 px of the top, from the open sidebar's edge to the right (the whole width when the sidebar is collapsed), with only the window buttons (and the update chip), and moves the window while down. The clear bar from feat: a clear title bar moves the window with vertical tabs, not the sidebar #189 is gone.
• Why the first pop-out bar (fix: title bar icon in outline, edge reveals that hold in a window, a hairline grip #185) could not be dragged, measured: it slid with a CSS transform, and Windows is told where a drag region is from the layout, so the region stayed where the slide started, above the window (WM_NCHITTEST answered HTCLIENT over the visible bar). It now slides by top; the collapsed sidebar slides by left for the same reason.
• The sidebar answers sooner: it shows within 24 px of the left edge (was 8), the bar within 16 px of the top; both after 60 ms (was 90). Main's cursor watch uses the same zones.
• A strip to grab above the tabs: 36 px, the bar's height, moves the window.

App only. 0.39.5 -> 0.39.6.

Checks

• e2e verticalTabs (45 checks): the bar out of sight and the terminal at full height; 12 px from the top brings it down; from the sidebar's edge to the right; window buttons only; Windows' hit test: the bar (while down) and the grab strip answer HTCAPTION, a tab and the terminal HTCLIENT, and the bar's row is the terminal's again once it is up; 20 px in from the left shows the sidebar. titleBar passes.
• unit 1902/1902; typecheck and lint clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_017XYVRPzvhLpPCr75uB6uaA