Partial merge breaks a separate stack based on an upstack branch

[ephess]

We created two GitHub Stack objects, where Stack B’s trunk was an open branch in Stack A. After partially squash-merging Stack A, GitHub automatically rebased its remaining PR but did not cascade the rebase into Stack B or warn that Stack B would be affected. The bottom PR in Stack B silently expanded from 4 files to 27 files, contradicting its description and confusing reviewers. Its effective merge delta remained only 4 files.

Expected: either cascade the rebase into dependent stacks, prevent/warn about cross-stack bases, or clearly mark the dependent stack as needing rebase before presenting a misleading diff.