Game window won't enlarge past a specific size on second monitor

I've come across an odd bug that's making it impossible to continue development: When I open a new game window/tab on my second display, I can't enlarge it past a specific resolution (1266px x 672px by my measurements).

I have had this issue before with the same project on another computer, and fixed it (IIRC) by deleting the player preferences ( PlayerPrefs.DeleteAll() ). I've tried that again, and have tried deleting the preferences saved in the Registry ( HKEY_CURRENT_USER/SOFTWARE/[Project Company]/[Project Name] ), to no avail.

Any thoughts on what might be causing this issue, and how to fix it?