In addition to the previous method, I found another new way to reproduce this bug on my computer.

:-] Hold my beer while I dismantle the three versions of Windows/Photoshop I downloaded and installed on VirtualBox yesterday.

On the plus side, I tracked down the issue.

The issue doesn't actually appear when I run Photoshop. However, when Photoshop starts up, mIRC redraws its windows, indicating that Photoshop has made a system-wide change. Long-story-short: a change made to mIRC a few versions of ago adjusts the MDI window layout when system settings change but only in certain conditions. It turns out that when an MDI window is already maximized and an API call is made to move/size it, this causes the MDI handler (which is managed by Windows itself) to malfunction and miscount the number of system/min/max/close buttons it needs to display. The only way to force the MDI handler to reset its count is by recreating the main menubar. This issue is also present in Windows XP.

In any case, I have made a change that should resolve this in the next beta.