This one is specific to Gnome shell 3. I like bottom panel of GNOME Shell, but its autohide feature is a turn-offs. There are a lots of extensions out there, but none provides this facility. You can help me by pointing to the right extension. (I am going through this https://live.gnome.org/GnomeShell/Extensions/StepByStepTutorial/ tutorial but I cannot say if I can finish working on this.