For example, in single-window mode, if I click somewhere inside the Layers dialog or the Tool Options dialog, then move the mouse pointer over the image while holding spacebar, the image doesn't move as expected. Even if I click on the image's tab at the top, the image still doesn't move.

PROBLEM: The only way to make the image move by holding spacebar and moving the pointer is to first click inside the image to make it active, then the spacebar+drag technique works. Unfortunately, this is an annoyance because then if I had the paint brush tool last active, clicking in the image to make it active means I made a paint mark in the image and it now has to be undone.

Further more, knowing that I have to click in the image to be able to drag it around, I now have to make sure I change my current tool to be sure I don't paint inside the image.

Overall, this is super annoying and is the only reason why I would not use single-window mode. If the user holds spacebar and drags the mouse pointer over the image, it should move the image around no matter what.

In multi-window mode, one simply makes the window active and can drag the image around, which is very easy and the user doesn't have to worry about making a mark inside the image. In single-window mode, it seems the only way to make the image active so we can scroll it would be to click on it's tab, but that is not the case. We should, at the very least, have the functionality where clicking the image's tab makes the image available for spacebar+dragging scrollability, though being able to do spacebar+drag for scrolling at all times would be better in my humble opinion.