inline comments break diff rendering when source lines are short

Steps to reproduce:
1. Select a file/diff with short source lines or adjust the browser width so that there is lots of whitespace between the rightmost edge of the source and the right side of the viewport.
2. Click on a source line to create an inline comment.

Expected:

• comment edit form pops up.

Actual:

• comment edit form pops up.
• the tetris, line number, and diff marker columns expand, throwing the alignment of the unifed diff out (see the screen shots).

The diff alignment is still broken after:

• immediately posting the inline comment, and
• page reload.