XFCE note: autohide panel (the clock is not), accessibility WM tweaks turned on (hide title...) as I usually have a window maximized. Raise-on-focus (Window Manager settings, similar setting in tweaks for window cycling) turned off, allowing rolled-up windows to not disappear when unfocused.
System note: ~~I have not maintained it well, broken dependencies right now and have finally got bit by nVidia as my system won't properly wake from suspend. Getting an alternative GPU is somewhat of a mess, especially prices and getting full performance.~~ EDIT: Updated, working fine now
I made this ultra-minimalist window theme a while ago (this is the second version, with the widget-capable layout and style-based color accent) and have been using it.
The title is 12px tall (the buttons are default 8px, though can get taller with alternate hover/click states).
At this size, XFWM has a design issue with font sizes/baselines so most fonts are cut off (the selected font is Nimbus Mono PS Bold 10, larger has text descenders cut because text can't overlap window contents)
I would try to take this idea further (and fix some of XFWM's other relevant issues) with my own WM but I use a somewhat niche language and couldn't figure out how to render a rectangle the last time I looked into it (the WM I was looking at doesn't have titles/window controls).
Not set on a name as lots of themes have size-based names but are not as minimal.
I think I'm glad it never released. Just look at those window control buttons.
Fair, I have no idea if there's much interest.
For difficulty, I would make wider+more spaced buttons if dynamic sizing were a thing (without it, small windows would have shorter titles when focused).
If you mean visually, yeah not many pixels and with this layout I cannot put a background behind the window buttons without it spanning to the title. The previous version has it but was right-justified, so cannot have the buttons disappear without the title shifting (the shown version allows longer titles when unfocused).
I think that design is viable if it's overhauled a lot. I'd make everything bigger, the color scheme more modern, the text actually visible and put all 3 window controls in one shape (like the title). Though it may not be possible without implementing heavyweight WM components.
It was too slow for an edit, but this is the older version screenshot (note:Imgur only loads links for me with a private window)
One small benefit of the current layout is that I can see both edges of a rolled-up window (when focused).
I would definitely try more things if I had more control. Big text would be nice with content overlap (or outside window+outlined+noBG?), inset controls especially if dynamic. Maybe even different placements of title or buttons.