The problem is not on the Factorio side of things, it is your touchpad's drivers, and it is not affected by Factorio at all (you are just noticing it more). You will need to use an external mouse.

And how does that explain the same issue on OS X, with the computer's stock trackpad?

Same issue, latest demo on OS X. I'm using the built-in touchpad of a Macbook Pro.

For me the issue is worse -- no left-clicks register for accessing inventory, no matter what I do.

Any updates on this? It's literally a game-breaking bug for me, and I just downloaded the demo to try this out...

I'm having this issue as well.

Running the latest demo on OS X. Couldn't get very far, since the campaign wants me to open an inventory immediately :/