NOTE: Minecraft 1.20 support will come in a follow up v1.12.1

Main Features:

1.17 support
Display editor: A gui for item names and lore via /itemfactory display. The name, lore, and hideflags item factories have been moved to /itemfactory display <name | lore | hideflags>
Book editor: A gui for custom written books via /itemfactory book [new]
Sign editor: A gui for custom signs via /itemfactory signboard [new <type>] (sign is already taken as an alias of signature)
Importing images (drag onto inventory for item with image lore, drag onto display editor to add to an existent item, drag onto book editor to embed into a page)
Tooltip overflow fix: Automatically moves and scales tooltips to stay within the screen, useful for viewing lore images (can be disabled in config)
Find & replace: Multi-line text fields (including the new formatted text fields) support Ctrl+F, including RegEx
No armor restriction: Allows placing non-armor items into armor slots (must be enabled in the config) - was already possible by editing armor items, but this makes it easier
Corrupt client chest pages are now backed up with a warning
/items has been removed in favor of the /open alias

Bug Fixes:

Fixed incompatibility with the clientcommands mod, causing a crash in some versions of Minecraft
Fixed command syntax errors showing %s instead of information
Fixed slots sometimes being referenced incorrectly (eg. space on armor opening a hotbar slot) in older Minecraft versions
Fixed the offhand slot being referenced incorrectly
Fixed /itemfactory display lore list, then clicking on a line, showing JSON instead of the currently configured text mode
Fixed the text format system not allowing multiple formats at the same time (eg. bold and italic)
Fixed item tooltips appearing underneath the page text field and logo in the client chest
Fixed crash when hitting enter in a string input field if the input is invalid
Fixed bottom right slot of a container being uneditable in some scenarios
Fixed the save indication not waiting for a parent save to finish (eg. a sub-container within the client chest should wait for the client chest page to finish saving)
Fixed client chest save errors not showing an error in chat
Fixed closing the client chest or container view right after moving something into your inventory creating a ghost item
Fixed deleting characters in multi-line text fields not adding an undo entry, so you couldn't Ctrl+Z just that edit
Fixed a warning in the console about CommandExecutionC2SPacketMixin when launching in older versions
Fixed the bridge library being loaded when launching the game causing a crash, making the mod incompatible with the feather launcher
Fixed expanding an opened string causing the string to be replaced with the non-string version (eg. NbtString '{"text":"test"}' => NbtCompound {"text":"test"})
Fixed crash when editing the display.Name tag if the name field at the top of the screen was scrolled over more than the new name length
Fixed text fields being edited from the bottom of the screen up causing the lower ones to appear focused in some guis
Fixed text field selection highlights not appearing on the text field in some guis
Fixed the cursor in some text fields not blinking

Noteworthy Code Base Changes (for API users):

ItemReference has been refactored, which allows you to create custom ItemReference types (meaning some enderchest mod could allow editing enderchest items directly for example)
ClientContainerScreen is now called ClientHandledScreen
the screens package has been sorted into sub-packages

NBT Editor

Item Editor

Client Chest

Putting shulker boxes inside a shulker box