JEI (Just Enough Items) and REI (Roughly Enough Items) are mods that are really useful, especially with other mods that adds a lot of crafting.

It could be nice to have full support for these mods on the backpack to allow a better user experience.
This is the conditions where no support is provided for the "move items" functionnality of JEI /REI :

• On Forge
  • JEI when the backpack is accessed through the inventory (right click or via the equipped backpack)
  • REI when the backpack is placed on the ground
  • REI when the backpack is accessed through the inventory (right click or via the equipped backpack)
• On Fabric
  • JEI when the backpack is placed on the ground
  • JEI when the backpack is accessed through the inventory (right click or via the equipped backpack)
  • REI when the backpack is placed on the ground
  • REI when the backpack is accessed through the inventory (right click or via the equipped

I should also mention that using JEI on the Farbic Loader is not ideal because custom recipes with mods like Create or Farmers Delight are not supported.

Forge has JEI support
Fabric has REI support

https://www.curseforge.com/minecraft/mc-mods/travelers-backpack-fabric/files/all
Added REI support for all Fabric versions

I quickly tested the update on fabric and it seems to work well.
Great job, and thanks for maintaining this great mod!