When I launch Spectacle and try to select a rectangular area, it seems to do something but there is no rectangular area visible and the screen around that area is not darkened. However, despite no rectangular area selected is visible, moving my mouse pointer around that area, mouse pointer changed its shape to triggers. So I tried to disable "Compositor" (Alt + Shift + F12) and ... voilá, there was my rectangular area selected and the darkened area surrounding it. So, there is some problem with Spectable and Compositor. I'm using OpenGL 3.1 render gear.
Can you still reproduce this issue with a newer version of Spectacle? I've never experienced such a bug with v18.04.
This is a problem with the graphics driver, graphics stack or Qt. There were no changes in Spectacle in that regard for the last releases. Closing this as a duplicate of a newer bug for now, because that one has more details already. *** This bug has been marked as a duplicate of bug 394499 ***
In my case, I can say any more about the issue, cause I changed completely my hardware and today I'm using OpenSUSE 15 without that problem. Sorry.