So, at first I could not reproduce this issue. However, I eventually did notice something strange. If I right-click in the same place each time, without moving the mouse, then the cursor starts alternating between the last place I put it and the center of the screen (not the window). The click from when the cursor is inappropriately shown in the center of the screen has no effect other than to move the cursor back to where I last left it.
However, if the center of the screen happens to be outside of your Guild Wars window, then clicking when it's there will take focus away from Guild Wars. Hmm, actually, it only seems to happen if there's another Windows program's window at that position. If there's just a Mac-native app's window, or the desktop, at that position, then Guild Wars doesn't seem to lose focus, although the cursor temporarily turns in to the Mac standard arrow and the click is still ignored. Very weird.
For what it's worth, I noticed this when testing on an iMac with ATI Radeon HD 2600, but not when testing on a Mac Pro with Nvidia GeForce 8800. It may be that I was just missing it on the Mac Pro, though. I can try retesting when I next have access to the Mac Pro.
I'll enter this into our bug tracker.