Hi Cameron, the TwoPaneSplitView class that Damian mentioned a few months ago (above in this thread) is now public. It is part of the latest UI Toolkit. See the following for more information:
- UI Toolkit 1.0-preview available for general info about the UITK package
- https://forum.unity.com/threads/ui-toolkit-latest-version-1-0-0-preview-6.941420 for the latest UITK package version
Hope that helps you,
Sebastien