If anyone would like to build their own floating UI panel, most of the backbone of SystemClock's clockUI is formed by DraggableContainer which is an extension of UIBox that has gone through multiple stages of development by Toeler, MathIsFun0 and I, and is in a very usable state as a drop-in module.
Toeler originally developed it as MoveableContainer with a large number of additional features for his HandPreview mod. MathIsFun0 then took an axe to it and cut it down to its minimal parts for use in Ankh Speedrun Tmer, which I then overhauled and built upon. It's possible that there are no lines of code left from Toeler's mod, but credit where it's due.
It can display child popups on hover provided in args.h_popup, can respond to mouse-hover and drag events in the same dynamic way that cards and sprites do with juice and zoom effects, and will work dynamically with other menus while paused.
clock_ui.reset() in src/clock_ui.lua provides an example of how it can be used. A mod that allows for custom popups with editable text probably wouldn't be too difficult, it's just not something I feel like spending time on right now