Enabling merge queue hides the stack interface

I just enabled the merge queue for my repo, and the stack interface went away. To confirm, I temporarily turned off the merge queue, and it put the stack interface back, so it seems clearly related to having the merge queue enabled.

Start by reproducing the issue in a repository with merge queue enabled, then compare the stack interface behavior with merge queue disabled. Trace where the interface is hidden and add a regression test showing that it remains available when merge queue is enabled.

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