Normally middle mouse button is used to lock items on toolbar.
If you map it to enter/leave ar this function stops working and there is no way to remap it to different button curently.
[0.15.3] Can't lock toolbar slot
Re: [0.15.3] Can't lock toolbar slot
GUI button interaction has to be mapped to a mouse button to function. The widgets (buttons) don't respond to any other input so the game doesn't know you're trying to interact with them.
If you want to get ahold of me I'm almost always on Discord.
Re: [0.15.3] Can't lock toolbar slot
Yep I managed to use one of additional mouse button for locking.