I really, really want my panel to auto-hide; I mean really hide. ... so I can't see it. Instead, I get a big, thick line in the place of the panel. I understand that the line helps newbies remember there's a panel there. (I don't know if there's a technical issue as well -- having a window there to receive mouse-over, for example.) (Still, how many newbies use an autohide panel? I imagine it's pretty close to zero.) I propose having the thickness of the "hidden panel marker" configurable, default would remain what it is now; but I could have a zero-thickness marker! I image there would be other people that would like a 1 px marker (liking the idea of a marker but not liking how very thick it is now). Note: this would also solve bug 2910.
i have a workaround for your situation. it's by no means a solution to this (IMHO) important issue, however it can make you comfortable meanwhile the problem is solved. set your panel to %100 transparency and make opaque on mouse hover. this way, border of the panel will be invisible, until you move your mouse over it. may be developers can easily solve this problem by putting a checkbox in panel settings dialog saying "hide panel border" which automatically applies this settings in the background. hence very little code change will be required. hope this helps:)
^ Well, that would make the background show, but it would still prevent application windows from using the space. And I don't think transparency works on weak video cards. I'll try it out, though. Thanks.
The devel panel has a (hidden) setting to disable struts (the thing that tels the window manager not to put windows under the panel). With struts disabled and the opacity on 100%, you won't notice the panel window until you hover it.
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.