The compact shell owns the back gesture and the install offer

Context

ADR-0040 made the browser build installable and ADR-0041 gave it a phone layout. Installed on Android, three platform behaviours still read as a website:

• The back gesture navigated the hash history. With a sheet or the settings dialog open it left the overlay on screen and moved the route underneath, and from the first route it left the app.
• Every tab bar tap pushed a history entry, so back replayed each tab visited before it could leave.
• The browser's install offer (beforeinstallprompt) went to the browser's own banner, while the storage warning told the user to install without giving them anywhere to do it. Chromium fires that event once per load and only shows the banner when nothing claims it.

Decision

Overlays in the compact shell each own one history entry. Opening a sheet, dialog, or palette pushes a state entry without changing the URL; the back gesture pops it and closes the overlay on top, leaving the route alone. Closing any other way pops the entry again, so the stack never carries an overlay that is no longer on screen. Two cases keep that honest: an overlay that closed under a newer navigation leaves its entry buried, and that entry is skipped when a later back lands on it; and an overlay opened while a pop is still in flight defers its push until the pop completes, because a push made before then would be undone by it. The recogniser is pure and unit tested (shared/ui/overlay-history.ts); the wiring lives in the sheet and the shared <dialog> primitive and applies only under the compact query, so a desktop browser's back keeps meaning navigation.

Tab bar taps replace the current entry instead of pushing one, the way a native tab bar switches destinations. replaceState fires no hashchange, so one is dispatched by hand and every hash subscriber sees the same event as a push.

Home-screen shortcuts and the share target are handled by launch capture (features/capture/launch-capture.ts), which reads its errand from the query string for the same reason: shortcut URLs are resolved before the renderer can read a hash. This decision leaves that mechanism as it is.

The renderer claims the install offer and puts it where the user already is: a one-line strip above the tab bar that closes for good on this profile once dismissed, the account menu, the Data settings section, and the storage warning itself, which gains an Install action when an offer is held. Safari never fires the event, so nothing may depend on the offer existing.

Dialogs are sized from the visual viewport rather than 100vh, and settings fills the phone edge to edge with the safe areas applied to the dialog itself, because a top-layer element sits outside the body padding that carries them. Under a coarse pointer the styled scrollbar is hidden so the platform's overlay indicator shows, and toasts sit above the tab bar and swipe away.

Every dialog pulls down to close on a phone, the way a native sheet does. The handle is a grabber drawn at the top under a coarse pointer, and the header where a dialog has one; only those two regions carry touch-action: none, so the body keeps scrolling and contains its overscroll, and the browser never claims the pull as a pan. The dialog follows the finger, closes past 96px through the native close so the caller sees the same event as Escape, and settles back otherwise. The threshold logic is pure and unit tested (shared/ui/dialog-pull.ts).

Consequences

Back closes what is on top, then leaves the app, on every phone that has a back gesture; iOS standalone has none and nothing changes there. A forward-navigation after closing an overlay lands on a skipped entry and is bounced back, which is accepted: forward has no native meaning inside an installed app.

A navigation that happens between an overlay's close and its history pop, in the same task, could be undone by that pop. The renderer's overlays navigate before they close, so the ordering holds today; a new overlay that navigates during unmount must keep it.

The compact e2e (apps/workspace/e2e/mobile-shell.mjs) drives the back gesture, the tab bar, and the settings dialog through CDP navigation history, which reports the current index where history.length cannot once forward entries exist.