This is a follow-on issue to my previous issue. I think they may be related because they both have to do with getting unexpected behaviour when dragging in layouts. I just upgrdaded to version 188.8.131.523 for Mac in case this has something to do with it.
Starting with a brand new layout in a brand new file, I try to increase or decrease the height of the body part by placing the cursor on the horizontal separator line at the bottom of the part and dragging downward. Sometimes it works as expected, but most of the time the body does not decrease in height, rather I get a black line that looks like a frame. Now, if I adjusy the width of the body, that works, but it leaves a black vertical line or "frame" on the screen. See screenshot below.
BTW, I can change the height of the parts just fine in the inspector but the black frame does not go away. If I go out of layout mode and return, the black frame is gone. But if I try adjusting the part size (this happens with any part) the frame lines comes back.