Which ones in particular?

Docking views is a bit complex, you'd need to implement that yourself. The rest is standard UI stuff. For animations look at scene2d actions.

VisUI is built on top of Scene2D.UI. It's great if you want a similar appearance to apps like VisEditor and libGDX Texture Packer GUI. However, if you want to make your own unique interface, I'd suggest staying with Scene2D.UI. VisUI has too many settings baked into the code to really customize. Just learn to create your own UI skins and widgets. Spine would not be dependent on a 3rd party library like that.

I created some example interfaces that might be of an interest you. I also wrote Skin Composer to make UI design in LibGDX a bit easier.

I believe I achieved that with my Skin Composer UI test. Window movement is smooth. Resizing is sluggish but works. It's been awhile, so I couldn't tell you what the trick was. All I can remember is that my code felt like a sloppy solution. Take a look at my example code and compare with your results.

It's definitely not as good as Spine's window. I would love to learn the team's secret in this regard.

No, my UI test is drawing a custom title bar. Real fancy looking, take a closer look. Using an undecorated LWJGL3 window.

My actual Skin Composer software uses a native title bar. I felt that the custom one was too quirky. For example, you lose the ability to "pin" the window to the corners of the screen under Windows 10. You lose that kind of functionality doing tricks like that. Spine suffers from this as well.

ImGui is an immediate mode UI library for C++. That is veeery different from libgdx's Java-based, stateful scene2d.ui. While I have not used ImGui, I'm not a fan of immediate mode for anything but simple UI and I certainly wouldn't create UI using C++. It can all be done of course but whenever we're talking about UI we're talking about degrees of pain, not whether it's possible to hammer a nail with a rock.

scene2d.ui solves a number of major problems that most (or all) UI toolkits have: sane layout, hierarchical transforms, events, low complexity. Especially having a solid approach to layout is crucial, since that is 90% of building UI. scene2d.ui is built on top of scene2d, which is at its core just a handful of classes. It's all very simple and easy to read code, making it easy to understand and customize. Don't like how buttons work? Create your own, possibly by copy/pasting existing classes and making changes.

libgdx also comes with the many benefits of Java, some of which are: fantastic tooling, code changes are hotswapped into a running app greatly reducing restarts, and a relatively straightforward and simple language that has been ruined by silly language feature fads. All this is true even though I am a libgdx author and I wrote most of scene2d.ui.

Nate Thank you for the detailed reply, its great appreciated! You did a great job with libGDX. JAVA does have awesome features. Since my application is in C++ it may not make sense to add another layer (JAVA<->C++ Bridge) and keep everything C++. This leaves me with trying to figure out which framework is more battle tested. Perhaps imgui with skia... :

There's a C++ port of libgdx, though I haven't used it:
https://github.com/aevum/libgdx-cpp
Looks like it's missing scene2d, so I suppose you'd need to port that. You could skip porting the JSON part of Skin. You still need Skin but don't need JSON, you can just configure your skin with code. I mostly do that these days anyway.

I really doubt imgui is the right choice for a large app, and I don't know about battle tested. Immediate mode is just so damn odd. Skia is low level, probably not what you want. Qt might be your best bet for stability and popularity.