Hi Sander,
Yes, I am aware of this problem. It is a hard-to-fix combination of peculiar behaviors in both Qt and PI's workspace. I have it identified and am thinking on the best way to fix it, but I decided not to delay 1.5.5 due to this issue, mainly since 1.5.5 fixes so many issues that are much more important than this one.
Actually, it isn't that the mouse becomes unresponsive (not at least in all test machines I have here). What happens is that the mouse cursor remains hidden over images while a popup window is visible and has the keyboard focus. For example, you select an item on the main menu and the corresponding popup menu opens over an image. Then if you move the cursor outside the popup and enter the image, the cursor hides. The effect of this event is that you feel that the mouse cursor suddenly ceases following your mouse device. However it does; it's just that you don't see it while it's over an image. Pretty ugly, I agree, but harmless.
As I've said I'm working on this problem. There are a few more ones that will be fixed in the upcoming 1.5.6 release
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