restoreView() refuses to reopen the popup ONTO a non-restorable view, but
the stack behind it was restored verbatim, so a screen the popup will not
render -- export-privkey, show-phrase -- could sit in it. Back then landed
on a view the popup had declined to restore, and show-phrase has no Back
control to leave by. No secret is exposed: those screens are empty
precisely because nothing is restored into them. This is navigation, not
disclosure.
loadState() now truncates the stored stack at the first entry outside
RESTORABLE_VIEWS rather than splicing that entry out, so the result stays a
prefix of what was stored and every surviving entry keeps exactly the Back
target it had; splicing would silently re-point the entry above the hole at
a different screen. A stack truncated to nothing under a restorable
non-root view gets main beneath it, so Back always has somewhere to go.
Stacks with no unrenderable entry are restored unchanged. A stored stack
that is not an array runs through the same rule instead of returning early,
so a corrupt stack does not fall back on the goBack() behaviour the
explicit main exists in order not to depend on.
The filter is on load, not on save: the live in-session stack is
legitimate, since the screen really is rendered while the popup is open,
and only a load-side filter also repairs the stacks already in storage,
including ones written before a view left the set.
Out of scope, tracked separately: a restorable view sitting in the stack is
unhidden by goBack() without being re-rendered, so it can still come up
blank on a freshly reopened popup. That is a general navigation defect,
independent of this filter, and is
#268.
