The problem from bug 1278581 was that hiding the URL bar in response to a
key-down event (for the Enter key) would then lead to the corresponding key-up
event then ending up in GeckoView, thereby confusing the "last user activity"
tracking.
It appears that this only happens with key events received through the
regular OnKeyListener, but not with events coming from the OnKeyPreImeListener.
On devices where pressing Enter in the URL bar would transmit the key event
through the latter mechanism, this means that because the key-up event we wanted
to suppress in BrowserApp never arrived, we would instead suppress whatever
other key event would arrive next, e.g. possibly a press of the back button.
This would lead to the observed behaviour where after entering an URL, the first
subsequent press of the back button might then be ignored.
Differential Revision: https://phabricator.services.mozilla.com/D30573
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