Blender Version
Broken: since 2.80

Short description of error
In the Shading Panel, the "Shadow" toggle and it's slider are faded when X-ray is on, though turning it on or off and changing it's value will influence X-Ray's Shading.
I don't know if the bug is either the "shadow" option being faded in the panel or if it is it influencing shading when X-ray is on.

Exact steps for others to reproduce the error
Open default cube
Turn on X-Ray
In the Shading panel, Turn Shadow On and Off. With it On, change it's darkness level and direction.