- Upgrade from 16.04 -> 17.04 LTS, Plasma 5.9.4

Since, a square container w/varying background hovers behind the cursor.
Maybe 30x30px.
Wherever anything actionable receives focus.
Seems to capture whatever is being hovered, just before any onhover style-change.
Typically partially obscures any tool-tip.
Especially unnerving whilst typing behind the window cursor e.g. here in this text box, where said container can completely block text behind.
Impossible to click/drag select accurately.
Cannot be captured via Spectacle but no-go.

Have tried changing themes, no-fix.

I have the same problem, also after upgrading from Kubuntu 16.04 to 16.10 and to 17.04.
My system has an Nvidia GeForce GTX 980M and I am using the proprietary nvidia driver.
If I switch to the noveau driver, the cursor/pointer seems to work as intended, no strange box behind it. However, I do need to use the nvidia driver to get access to some of its features.
I am not sure, however, if this is a problem related to nvidia driver interacting with KDE Plasma OR an issue of it interacting with whatever noveau configurations.
It's not easy to describe this problem in a short sentence, thus it's very difficult to find information about it, because people might use anything from "mouse problem" to "pointer issue", etc.
Can anyone provide any clues for troubleshooting and/or workaround? This is really a show stopper bug.

After unsuccessful search in KDE bugtracking system, I opened the following bug report. Please check and contribute there.
Bug 381533 - Square box behind mouse cursor/pointer blocks and corrupts zones at which it's pointing