What version(s) of WoW are you using?

Classic Era

What problem are you experiencing that led to you asking for this feature?

Often when wanting to split a stack and give someone a handful of them instead I need to open trade or change the options to see the split - preventing just picking up and dropping the split item stack to open trade.

Clicking through the Options > Layout > Group checkbox or swapping to Single Bag view to just toggle back/forth feels clunky.

What solution would you like?

When Category view is enabled; add a button next to the "Bag Slots" button that toggles the Group identical items button in the options.

Any alternatives you can think of?

An option to display recent & split items in the Recent category as ungrouped.
A keybind to toggle the option,
Maybe a button/click action for each category to also toggle the Group identical items option, such as for Ammo or Consumable.

Anything else?

None

There's no plans to add an explicit "Toggle Group Identical Items" button.