I suffer from the same problem, but apparently only since about Wednesday (in this extent).
One think that significantly helps (from a Steam forum post):
- Select a character portait within the game with no Ui open (left or right dependent on your UI layout)
- right click on the AI - button which hovers atop the party portraits
- the bahviour editor opens and e.g. condition selector items should be clickable
Iopen e.g. the Inventory
select a character
click the behaviour button below the portrait
click on "Edit behaviour"
click on a existing behaviour or create a new one
click on a condition (e.g. "always true" on a new bwehaviour set)
you are only able to click some of the items (upper two items, some in the middle), scroll bar ist not always working.
I guess the underlying window is still active somehow. But (in contrast to Pillars I) Ui Windows are not freely moveable. Reducing font size does not reduce UI size.
I tried to change "Windowed" to "Fullscreen" and lowered font size without success. However, above workaround to open AI Editor from party panel worked.