I experience the same problem, I have found with 1.14 the issue could be resolved by quitting and restarting the launcher. I have not been able to get 1.15 to work in this manner, however, I have found that I can resolve the issue by starting 1.14 first, and if single-click works, I can then quit and restart 1.15, and single click works as expected. This does not resolve the issue, but it is a work-around and perhaps it can help some of you who experience the same problem.
I had to replace the SSD on my MacBook Pro, and I forgot to save the directory
$HOME/Library/Application Support/minecraft
. (Fortunately I had a backup of my main world on another computer.)So I had to reconfigure Minecraft, and log in again. However, after I have done that, my mouse-clicking problem has gone away.
I suggest that everyone having this problem make a backup of the
saves
folder, then delete$HOME/Library/Application Support/minecraft
and reinstall from scratch. Then put back thesaves
folder where it belongs.