you say this because of the
"on the left handside number of the sprdsheet"

you are right and wrong at the same time: because in your case the macro
will open anywhere in the row since the entire row has been selected first.
if you select the row (left clic on the number) then right clic
anywhere in the row, then YOUR macro is executed and shows the "Aha"
message... This is not really a solution neither.

I'm not sure there's a (simple) way to be sure the clic was on the left.

This mean Broogle can have a macro lauched only when the clic is in
the row, (with my code wiht no need to select first the entire row, with
yours, need to select the entire row but may be before)

My and neither your code guaranty only the "left number in the margin"
clic asked !

I do not know the way to really check the position of the mouse when
the clic occurs. But if there's a solution, he needs to look at the
mouse coordinates and check these are at the left of the left cell of
this row (after having my or your way of selection, one more test).