Cross posting again a post on the Technical Support & Bugs forum for a development aspect…
If a modifier is needed on Left-Click to support Flare, then I am pretty sure that it should not be Control on MacOS because on that platform Control is the modifier that turns a Left-Click into a Right-Click.
Command and Shift are also ruled out due to their existing select/de-select roles (both issues addressed in the parent of this thread) but it occurs to me that flare might be combined with each of select functions as follows:-
Alt+Click:like Click but Flares the item clicked on
Shift+Alt+Click: like Shift-Click but Flares everything that is selected currently.
Command+Alt+Click: like Command-Click but Flares the item that is being selected or de-selected
Aside: Alt-Double-Click would be good standard choice as a mouse-click zoom-out function (discussed in the Zoom Standards thread). So, the above partly assumes that Alt-Click is distinguishable from Alt-Double-Click.
If the above is way beyond the requirement/scope of “Flare” and/or the Alt is unsuitable, can the Fn key be combined with a mouse-click ?