Reported from the first device run: back does not navigate back in the
app. The scaffold's `MainActivity.onBackPressed` asks
`webView.canGoBack()` and finishes the activity otherwise -- and this
app had never touched `history`, so that was false at every depth and
back quit from anywhere.
The fix is here rather than in Java, because the mechanism the scaffold
already uses is the one we were failing to feed: a navigation is a
history entry now, and `popstate` replays it. Nothing on the Android
side changes, and the behaviour becomes assertable in a browser with
`page.goBack()` instead of only on a phone.
The entry keeps the same URL -- the app has no routes, and a path a
reload cannot resolve is worse than none -- and carries the destination
in its state.
Two rules keep the stacks from disagreeing. The first navigation
*replaces* the launch entry rather than pushing one, or every launch
costs a back press before the app will close. And the in-app back
buttons go through `history.back()` rather than popping a stack of
their own: `navStack` is deleted, not kept alongside, because two
stacks is precisely how a detail view's own button and the phone's
gesture come to disagree about how far one press goes. The third spec
pins that invariant.
logan
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