in the manual it says

Synplant automatically triggers a note to play it whenever you click and drag a branch. (If this is undesirable, turn it off using the MIDI Settings window.)

to which menu point this is referring to? also tried several options. can't find it.

Easy to miss, this is from the manual regarding MIDI settings.

Output Channel
Choose the MIDI channel Synplant will use to send out MIDI notes when you play
branches from the user interface. Setting this to Off will disable note playback when
editing branches.