You can add an movement block to the backdrop by moving a script into the backdrop.

Recreate:
Step 1) Open a project
Step 2) Add movement blocks (may be mixed with different kinds of blocks like variable controlling blocks)
Step 3) Move the script to the backdrop by taking it (you hold it), then move the mouse/your finger to the part which opens the backdrop. Then you release the mouse/your finger.

These block won't do anything in 3.0 (don't know about 2.0), so it is not very urgent but it is weird. (If you need to wait until a block finishes use this if in the backdrop instead of

wait (0) secs

My browser / operating system: Windows NT 10.0, Chrome 84.0.4147.135, no Flash Version detected.

In 1.4, the project seemed to detect this, as there would be a red outline around the block when you clicked it, rather than a white one.