No actions in admin (actions=None) messes up layout.
When a changelist page on the admin has no actions to display and therefore no checkboxes to put in the first column of each row, the layout of the columns is mangled. This is because the styles in changelists.css use the first-child selector and set the width of that first column to 1.5em. This is fine when it is indeed a checkbox column, but makes it rather difficult to fit in text of any reasonable size when it is not.
The attached patch solves the problem by setting a special class attribute 'action-checkbox-column' on the header for the action checkboxes and the width of 1.5em only applies to that class. There are also two simple tests.
This bug arises because of an incomplete implementation of ticket #10595.
Patch with fix and tests