I tried giving single left clicks two different meanings. If they occurred in a field that already contained the cursor, they meant "type the LButton character." But if they occurred in a field without the cursor, they meant "move the cursor here."

The result was bewildering. Even though I knew what was happening, it seemed as if the program was broken.

I think the idea of moving the cursor by left clicking is so natural and familiar, that it's just not possible to mess with it without confusing people.