Double click on a tab bar to dock is pretty a standard behavior.

For information, on Linux, there is a "dock" icon which matches the "undock" icon (windows/linux) which is on the bar when the module panel is docked.

We could enforce this "dock" button to appear on Windows and Mac. Again, we are probably breaking the operating system Look&Feel guidelines: why would Slicer be the only app on a Mac to have that icon for an undocked tool panel ?
But I don't have too strong feeling about it.

For the one who implements it:
http://doc.qt.nokia.com/latest/qdockwidget.html#setTitleBarWidget

Otherwise, why not using the "pin" icon ?

Wendy:

For something like docking behavior, which often confuses the life
out of beginner and intermediate users, I think we should be as
clear as possible... it's easy to get the interface into a configuration
that you don't want.

I can imagine in tutorials that someone will say "uh, i lost the GUI panel!"
or "how do i reattach it?" and then we want a quick and easy answer.
(I have no idea what Linux dock/undock looks like, so I wouldn't know
how to answer -- tho I could be trained!)

I vote for having a cross-platform same solution (which is clearly
distinct from the platform-specific dock/undock symbols on any supported
platform).

Alternatively, we could use the platform specific icons AND a Slicer icon
too, so we can make novices AND platform-experts happy.

This is requested by a user, see https://discourse.slicer.org/t/in-documentation-wiki-refering-to-undocking-docking-what-is-the-panel-chrome/1720/2