Merges can be staged even if the commits they include can not

See https://codereview.qt-project.org/c/test/project/+/280377/1 for an example merge. It merges a commit on another branch into master. That commit is under review in https://codereview.qt-project.org/c/test/project/+/280376 but has not been accepted.

If I try to submit it, then I get an error message:

Could not perform action: Failed to submit 1 change due to the following problems:
Change 280377: Depends on change that was not submitted. Commit b3d67e86d59a662814e569ee1c42631adf218804 depends on commit 1b6ef0382891e05cd9865a31afa85d9f43c05fe7 of change 280376 which cannot be merged.

This is correct, because I shouldn't be merging stuff that hasn't gone through code review. I should not even be pushing merges for review that include changes that are not in origin yet, but nothing prevents me from doing so.

However, staging still succeeds. CI will attempt to build the merge commit. Expected behavior is that staging would fail for the same reason.

(I'm marking this as visible internally only, since it sort of describes a way to get unreviewed content in. Of course the merge still needs to be reviewed, but the odds that reviewer would miss an unmerged change inside the merge are high.)