@lenemter this looks really cool!
A few things:

• I would prefer if we keep a separation of concern between generic shell windows and panelwindows. For example centered windows like the polkit dialog, etc. are shellwindows and they don't require all of the hide stuff. I proposed #2563 to move the visible in multitasking view to the panel window where it belongs. This could also be used to easily plug in this PR with only changes to the panel window (via the virtual method). Let me know what you think :)
• Instead of introducing a CUSTOM_2 I think we should introduce a DOCK for the controller that handles the progress started by the user, i.e. via the barrier and the simple gesture. Because in the future we actually want to use the gesture controller instead of the simple gesture (I hopefully find time to do this in the near future) and then we'll need it anyways.
• Since this is an area with lots of factors that caused a bunch of problems in the past with edge cases we missed, we might want to split this into multiple commits. For example a commit to split the hide tracker for user overwrite (i.e. barrier and gesture) and the hidetracker for the current state of the workspace and then a commit that actually hooks up the workspace switch.