Where is the source code for UI Toolkit?

Where is the source code for UI Toolkit/UI Elements, (com.unity.ui)
It used to be public on Unity's BitBucket https://web.archive.org/web/20160328041925/https://bitbucket.org/Unity-Technologies/ui/src

Where is it now, its not on the GitHub?

 

Sure I can "see code" but it's 10000% more useful if you can set BP's and even tweak/edit code. When/if we run into bugs, it's super useful to be able to debug the actual issue at a low-level, so that we can report it, and maybe then "work around it" until a fix is deployed into the next version. etc, etc...

As is, seeing the code is not really helpful at all, when/if anything goes wrong, or we get confused about how things are supposed to work. Gotta be able to debug it (with breakpoints) at minimum.

Got tips/hints? We're considering changing our application from Xamarin Forms over to Unity3D, but need to know the UI is not going to give us major issues.

 

Stumbled upon this thread. As UITK is now part of Engine (not a separate package) it's available on Unity source GitHub: https://github.com/Unity-Technologies/UnityCsReference/tree/master/ModuleOverrides/com.unity.ui