Thanks for the reply. All my controls were at the default values; so I couldn't press the reset button. It was grayed out. I went ahead and changed one random control, then pressed the reset button, and now it works. Thank you for your help.

Hi there,
I recently opted into the latest experimental version of the game, which is 0.17.66.
I like the new UI except for one thing. Whenever I use the left mouse button to drop an item or stack of items into my inventory, the game thinks I'm using the left mouse button to build the item instead ...