I got it to build on my Ubuntu Mate 16.04 system. Here are my observations:

* Custom Gtk::Range widget couple with Gtk::Spinbutton:

  This is my current favorite. It works quite well, methinks. Use the
'Alt' key to get into fine adjustment mode. It was rather simple to
implement. Could be refined further (add styling properties, rounded
corners, etc.).

With a white background and 1px dark grey border this looks pretty good. It does, at least, then look like a single composite widget, rather than a group of separate widgets. I am seeing major redraw problems with the bar, however.

I'm not convinced by Alt to get into fine adjustment mode. A lot of Linux window managers use Alt-drag to move windows around by default - this is already a problem in some areas of Inkscape use, but having to work around it for every slider in the application would be a pain. Perhaps use Ctrl or Shift instead?

(I also notice that pressing Alt when already dragging sets the value to 0, rather than switching to "fine control" from the current position. Not a concern for this test, but worth bearing in mind for the final thing)

* Gtk::Scrollbar coupled with Gtk::Spinbutton:

I'm not keen on this one, as the cognitive dissonance in it looking like a scroll bar but not actually scrolling anything is a bit too much for my meagre brain to handle. Perhaps if CSS can make it look a little less scrollbar-like it might be worth pursuing, but otherwise I think it will confuse more than it helps.


* Gkt::ProgressBar coupled with Gtk::Spinbutton:

As you mention, has the potential to look pretty good with some CSS, but with the non-functioning progress bar at the moment it's hard to tell how nice it will be in use.

* Subclassed Gtk::Spinbutton:

Up there with the first one in terms of looks, but doesn't suffer the same redraw problems. Issues with the size of the text widget though, as you noted.


* Gtk::Scale coupled with Gtk::Spinbutton:

Not too bad once I added a white background and a border to make it look more like a single composite widget. Not as obvious as the ones with a full-height bar, though. This is probably very subject to the whims of your GTK theme, however, as the thumb on mine is nothing like the circle in your screenshots (mine's a rectangle, for a start).

IMO the first one comes pretty close to what I would like to see, if the redraw issues can be addressed. Otherwise the fourth one is the best (though with issues around the value field), with the fifth one following in third place.