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 is somewhat of a mess, especially prices and getting full performance.
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.
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.