Hiding the chrome in the frame between the post-frame opaque mark and the
next build trusted an opaque flag the renderer never realized: hide() kept
controlsPresented while the fade-in target never rendered, so no fade-out
ran, markControlsHidden never arrived, and Back/Escape stayed swallowed
until a later show/hide cycle repaired the flag.
The controller's opaque flag now follows the real fade-in completion via
AnimatedOpacity.onEnd, so any hide it observes can rely on a fade-out that
will actually run. Branches that mount the chrome directly at full opacity
keep marking immediately, since those insertions fire no onEnd.
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