Regression - doesn't happen in 2017.3.0f1 and 2018.1.0a5, started to happen in 2018.1.0a6 and later (also in 2018.1.0b5)
I am on Windows 7 64b

How we can reproduce it
1 Open Unity and Create new project
2 Drag and drop any .png image (for example attached one - testimage.png)
(or you can simply open attached project)
3 Select image in Project view
4 Go to Inspector and in Preview window press button for inverting colors twice, then click LeftMouseButton on "Texture Type" dropdown to unfold it
- please see attached video for details

Actual results:
Unfolding dropdown performs "Undo" action and image in Preview is inverting its colours

Expected results:
Unfolding dropdown should not perform "Undo" action