Modicum

Although this list is small, many more are planned. All features can be disabled in the config file.
Hotbar Armor Swapping: Right click a piece of armor in the hotbar while another piece is equipped to swap between them.

Cauldron Dispenser Behavior: Dispensers pointing at a cauldron can transfer water in and out of it, using bottles or buckets.
Jukebox Dispenser Behavior: Dispensers pointing at a jukebox can put discs in it. If a disc is playing when another disc is inserted, the old disc will be put in the dispenser.
Rail Scaffolding Behavior: Rails can be placed like scaffolding, where you right click on one block to add more in the direction you're facing. This even works when going up and down slopes.

Jukebox Particles: Like in Bedrock edition, jukeboxes with a disc in them emit note particles.
TNT Dropping: If you have both TNT and either flint and steel or a fire charge in your hands, you can place a TNT entity midair by right clicking. Useful for elytra bombing, and can only be used every 3 seconds.
TNT Defusing: If you click on lit TNT with shears, it'll defuse and disappear.
No Potion Shift: Stops potion effect icons from shifting the inventory to the right. (Because of an event added to Forge this is a one-line change and is probably done in other mods too. I wasn't sure which had already implemented it so I just added it to this one. In any case, it won't break if more than one mod stops the GUI from shifting)