Just to rule out possible causes, is there a different mouse available so you can swap them and see if you get the same problem with either mouse?
Is there a different computer available where you can try this?
Have tried this with more than just one file?
Thank you for your post.
If you click the rectangle tool once, it will disengage after drawing a rectangle. If you double-click the rectangle tool, then it will remain active so you can draw multiple rectangles. Just click on another tool to make it active, or just select the arrow tool to return to normal state.
To answer PhilModjunk's question, the problem seems to occur across different files. Although I have not tried to reproduce this problem on another computer, it does happen with another mouse. On further testing, the problem is not just with the rectangle tool, but with any drawing tool. Somehow, although the FM window appears to be active and FM is listed as the active application on the Menu bar, the FM window is not active and I'm really creating a line or object (rectangle or circle) from within Finder, not FM. The workaround is simply to click on the FM icon in the dock to make the window truly active. My guess is that the problem is with my Mac, not FM.
Thanks to PhilModjunk and TSGal for responding; the problem does not appear to be with FM.