Vassal V3.5.x on MacOS Big Sur, using trackpad (need to re-test with mouse).
Line of sight tool ctrl-left-click-drag to share with opponent does not work on MacOS.
The same module, using the same click-drag procedure works from Windows to MacOS but not when initiated by the player using MacOS.
The bug is only apparent when online with another player.
This is probably because Ctrl+Left Click is interpreted as Right-Click on a Mac (cannot be changed by standard tools).
Command + Left Click might be an appropriate alternative for MacOS.