This tends to happen when your juggling layout of several elements and then you find you cant select one element. That element that cant be selected has been moved outside its bounding box. The visual element you can see can not be selected. If you can find the bounding box you can select it but if not you are stuck. Select all usually helps find the bounding box. See image below.
While the element is in this state changes have no effect until it is out of this state. You can fix this by finding the bounding box and resizing…sometimes but closing that page/panel and reopening seems to fix it. Maybee its something to do with graphic rendering?
This bug has only shown up in the latest version of Rp 8.1.0.3369. I have this issue on my office and home PC’s both running windows 10, (probably latest version as auto updates ate on)