#5135 Floating panes discussion

This is issue is for discussion of the floating panes, for questions about how they work and to collect and discuss feature requests and development notes. Please do not use it as a substitute for opening issues or PRs for bug reports or code improvements.

Floating panes are panes which sit above the layout ("tiled panes") like popups but unlike popups are not modal and behave like panes (so the same escape sequence support). Floating panes are created with the new-pane command, bound to * by default. They were mostly written by @mgrant0 with help from @daneofmanythings.

As of c9e22ab67bd7fe89ebb767215f2e671e72573e00 floating panes are enabled in master. Currently they can only be moved and resized using the mouse. The default second status line (if "status-format" is set to 2) has been changed to show a list of panes, and if pane-border-status is set to top or bottom a couple of controls are available on the right to close and zoom or unzoom.

The plan now is that the what is in master now plus small features and bug fixes will be in 3.7, additional features will be added after that with the aim to get them into 3.8. At some point popups will either be removed entirely or changed so they cannot run commands (that is, they would be to display text only).

Development on this feature continues in the floating_panes branch, feel free to test this and report issues but note that it may be unstable or the behaviour may change before reaching master.

This is a list of features or ideas for features which are currently missing:

Ability to move a pane between floating and tiled (break-pane/join-pane/move-pane).
A flag to new-session to tell it the initial pane should be floating rather than tiled.
What happens with floating panes on resize needs to be decided. Resize hints (keep in position, minimum size, etc) would be useful, but this may be better as part of a wider layout rework since some could equally well apply to tiled panes.
Support for custom layouts with with floating panes
A way to hide and show floating panes ("hidden panes").
Support for SIXEL with floating panes.

Support for some of these in in progress in the floating_panes branch.