Picking breaks if camera far (Clipping End) is set too large.
For example, create a new project (800x800 presentation), add a cube, and move it to the corner. Add a sphere and leave it on origin. Set layer camera clipping end to 100000. Now clicking anywhere on the scene view will select the sphere. Move the sphere halfway off-center on some axis, and nothing can be selected anymore. Widget drag also doesn't work correctly anymore.
This issue blocks having reasonable far clip on perpective camera.