Pressing the spacebar immediately after clicking anywhere in the frames/layers bar does not trigger pan image mode. My guess is that the frames/layers bar has focus and that the image editor doesn't know the input is "for it," but I'm not sure if that's how ASEprite works. ;)

In any case, if I switch layers and then go to pan using spacebar, whatever tool I currently have selected (not pan) is invoked instead, so often I'll switch layers and then go to pan the image, but get a nice, long brush streak over my graphic when I wanted to scroll it instead. After any click or (probably any) non-spacebar hotkey invoke in the editor window, spacebar-pan works as normal.