DisplayFusion 3.1.8.107 (beta 7)
DisplayFusion is causing
http://store.steampowered.com/about/ to ignore the resolution of the secondary monitor when it is maximised if the secondary monitor is a lower resolution (I do not know what happens in the reverse scenario, where the secondary monitor is a higher resolution).
In my case, the primary monitor (left) uses 1920x1200 as its resolution while the secondary monitor (right, bottom-aligned) uses 1680x1050 as its resolution. When maximising Steam on the secondary monitor with a DisplayFusion taskbar active Steam will maximise as though it was attempting to fill the 1920x1200 monitor and thus has a section of the right/bottom edges of the application cut-off. This issue occurs with other Steam windows as well (such as the Friends list); it appears that all of these windows belong to the one Steam.exe (they don't spawn processes of their own, if that's useful).
The only workarounds I have discovered are to not maximise Steam on the secondary monitor (smaller) or closing DisplayFusion to disable the taskbar.
Note: Using a third-party image host because attaching the 560KiB file (despite the 5MB limit) gave me the error "PHP Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 1920 bytes) in C:\Web\BinaryFortress\Root\Forum\Sources\Subs-Graphics.php on line 389". Good thing I wrote my post in notepad or I'd have lost it a few times.
Scroll right to see the impact, if your resolution is low enough to cause image scrolling.