Hi Chris,
Thanks for that succinct description of the correct procedure. Everything you describe is exactly what I have been doing for several years. I use Chrome, always organise the blocks in a single column, right-click in a block well away to the left of the blocks, release the right click, so the drop-down command list stays on the screen, and then left-click on the drop-down list to select a command. The problem is that the drop-down box doesn't always stay on the screen and when it vanishes it selects an unwanted command.
The method seems to have changed. What works now is to hold the right-click-button down to display the drop-down list, keep holding it down while positioning the cursor over the desired command, and select the command by releasing the right-click-button. This works for me whether using a mouse or a touchpad.
I changed to Windows 11 before restarting to use App Inventor and I can only think the drivers to the mouse and touchpad have been altered in the Windows 11 update.
Your comments were reassuring as for a while I thought that I might have been using an incorrect procedure all these years.