Advanced Mode: signing in from a cold auth wall on Android can never reach the server

[joemasilotti]

In Advanced Mode on Android, signing in from a cold auth wall can silently do nothing. The form submits and no request ever reaches the server.

Logcat from a captured run: the wall is visible, the identity check passes, the sign-in submit executes, and then nothing at all. No POST, no session, no visit, 30 seconds of silence. Closing and relaunching within the same session fails the same way.

Cause

The sign-out teardown runs resetAllTabs and its snapshot while the promoted auth-wall modal is still live. That resets the visible wall's Turbo session underneath it, so when the form submits it proposes a visit into a navigator that no longer exists. Same family as the ModalRootGuard work in d421536, and it affects the legacy teardown and the identity teardown alike, since both run the same reset actions.

Reproducing

Order-dependent, which is why it came and went in casual use. It now reproduces deterministically by running a single DocsAdvancedInstrumentedTest method in isolation: the test opens a cold wall, submits, and fails with "Tab bar never appeared."

Running one method at a time is the tightest loop available for the fix, so this wants a focused device pass rather than a blind change. Not a 0.12.0 regression: the same failure was A/B verified against a pre-hardening commit.

[joemasilotti]

One thing to watch for while driving Advanced Mode on a device for this: whether the nav bar visibly shifts when tapping between navbar segments.

Segment taps route a VisitOptions(REPLACE) visit through the navigator, which swaps the Hotwire fragment and rebuilds the per-fragment toolbar, so the segments should pop in only once the new page sends its first report. That would read as a wiggle. It is inference from the code and has never been observed, which is why it is a note here rather than its own issue (see #95).

If it does show up, the fix direction is seeding the incoming fragment's chrome from the previous report instead of waiting for the new page to report.