Patent, not copyright.

A patent is protection of a behaviour, or technology, or operation, that is unique. It has specific criteria to ensure uniqueness. Say, you patent a mouse trap - the behaviour/operation has to be unique. If it's the standard "lever with bait activates a spring loaded arm, capturing the rodent" type, no matter how you shine it up, it won't pass the uniqueness test, therefore not be granted its own patent.

A copyright applies to pretty much anything you can write. I could copyright this comment, for example (but then run into the issue of publishing it publicly on social media, thus needing to grant the right of reproduction to Lemmy/Mbin/etc. instances, so let's not get into that discussion). It doesn't have to be unique as long as you can prove you've come up with it originally. Technically, two writers could come up with the same book, and as long as they have evidence of writing it individually, both could get the copyright.

So overall, you could protect the functionality of left click via a patent (something along the lines of "operation of a digital computer or equivalent using a two-or-more-button handheld positioning peripheral" that covers pretty much any mice and any device you can connect a mouse to), but it would expire quickly (IIRC 20 years), unlike a copyright does (which is usually author's death + 70 years, unless author has a continued rights holder set up after the death, or prior to their death commit their work to the public domain).