When you propose a branch for merging, the proposal gets its own page, along with six secondary pages. Four of the six -- "Edit branch merge proposal", "Request review", "Set proposal as work in progress", and "Resubmit proposal" -- consist only of the Whiteboard field and some buttons. "Mark as merged" contains even less, a single-line field and buttons. And "Delete branch merge proposal" consists of two buttons and nothing else.
Having these as separate tiny pages is inefficient, but in the last case it's also confusing: I'm apparently required to choose whether to "Delete" or to "Cancel" a branch merge proposal, without any explanation of how those actions differ.
I suggest that all these pages be merged into the proposal page:
* "Edit branch merge proposal" become a JavaScript-powered button that makes the whiteboard editable;
* "Request review" become checking a checkbox;
* "Set proposal as work as progress" become unchecking the same checkbox;
* "Resubmit proposal" become ... something (I don't understand what this page does);
* "Mark as merged" become a collapsed-by-default section containing a text field;
* "Delete branch merge proposal" become a "Withdraw Proposal" button near the top of the proposal page.
Fixing this bug would incidentally fix bug 202015.
A sketch would be helpful here.
Note that these pages are new and are a good candidate for the updated UI.