Not sure where that idiom comes from or why strings work in some places
(auto-coercion?). I picked it up off some example apps. But
https://love2d.org/wiki/love.mouse.isDown says it should be an integer.
Problem: repeatedly copying (relatively large) sections of text quickly
makes the app sluggish until it has to be killed. (Thanks John Blommers
for the report.)
When I instrument with prints, the sluggishness seems to happen in
random draw() calls many times after I perform the copy.
I don't know for sure, but I'm initially checking if the cause is
garbage generated by repeated string concatenation.
This attempt doesn't seem to make any difference.
