bug(ios): system keyboard sometimes blank after device unlock

[jahorton]

From general testing of #11179.

Test Results

• TEST_GENERAL_USE (FAILED): Retested with the attached PR build (Keyman 17.0.305-beta-test-11179) on an iPhone 13 Mobile device (iOS 17.4) and here is my observation: 1. Set Keyman as the system wide keyboard. 2. Opened Safari browser. 3. Switched to Keyman Keyboard. 4. Typed a text on the Search bar. 5. Locked the mobile device by pressing Lock key from the mobile. 6. Unlock the mobile. Here, I noticed that the Keyman Keyboard was no longer visible in the Keyboard panel. Seems to be an issue.
  

Interesting that we haven't seen this one until now. There always have been odd interactions with that specific use case - see #2300 - and I can see the host-page getting reloaded / restored after locking and unlocking. It's temporarily blank for me, rather than permanently blank... but I'm also not sitting and waiting between locking & unlocking. If something were to prevent the host-page reload, that could certainly cause the issue. Of course, getting evidence of that is an entirely different matter.

Originally posted by @jahorton in #11179 (comment)

It's hard to say that this is a Safari-specific interaction, of course, but #2300 existing does make it a possibility in my mind.

[jahorton]

It may not be fully related, but aspects of this remind me of #6191.

[jahorton]

Note that the blank keyboard seen here was after the following instruction/reminder:

As for the third user test, please verify that "Allow Full Access" has been toggled - that would probably affect the system keyboard issue you saw.