@trace-calvin
XButton1 - 4th mouse button. Typically performs the same function as Browser_Back.
XButton2 - 5th mouse button. Typically performs the same function as Browser_Forward.
So NPP cycles back through tabs on both Browser_back event and Xbutton1 event. I didn’t find any option to disable or change it. If it bothers, you can use Autohotkey script to change this button, or block completely with this script:
#If winActive("ahk_exe notepad++.exe") XButton1::return