It would be cool to be able to resize the panel to arbitrary width not only to normal and full width
Yes, I guess this would be nice. It's probably not as easy as it sounds, though. Sizing and positioning have always been tricky areas with lots of subtle interactions.
4.8 panel will have a slide to set the panel length in percentages. 100% will result in the same effect as an expanded panel.
Devel branch has been merged in master. A 4.7.0 release will follow soon. If you think this bug is not fixed? Feel free to reopen the bug.