Cannot open gamemode switcher while in a nether portal

Can confirm. It is actually possible to switch game mode during the brief time it pops up, although it is difficult to do.

It would be expected that either it stays open and allows you to change your game mode or it would not open at all. The former is preferable, since the debug options menu (F3+F6) works in a nether portal just fine.

I will request that MC-225996 be re-resolved into this rather than MC-2071.

Thank you for your report!
After consideration, the issue is being closed as Invalid.

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I can’t agree with this resolution. The present behavior is very clearly not desirable (either the switcher should remain open or it should not open at all), and it would have required minimal effort to refocus this report on the currently observed result (i.e. change the title and description).