I have tried adding it via keyboard > shortcut keys bit in my case (wanting to move to next tab) it opens the tab but splits the screen, which is indeed not what I need... Maybe it works in your case.
Also, I looking into "XCODE CUSTOM KEY BINDINGS" try googling it, you will find it is possible to edit "Default.idekeybindings" as a text file and add what you need. I have found that (under the Shortcut key tag):
^ is equal to control (of course)
$ is equal to shift
@ is equal to cmd
and most importantly:
~ is equal to option.
Other special keypresses may show up as a question mark (inside a square) in the my editor (sublime text), maybe due to encoding. I would assign something I Xcode and quit and then check the above file, then modify.
I don't know why, but researching this, the option key started working for me. To find the code for the option key, I chose a random action (Export...) in the Xcode key binding prefs and tried assigning option+s to it and it worked - I assumed because it's a simple shortcut, but it turns out that where I need it (Show Previous Tab and Show Next Tab) I am unable to use the option key.
So what I did was assign the shortcut key I wanted to "Export..." and something wrong to "Show Previous Tab", exit Xcode, go to Default.idekeybindings and swap them. Then it would act exactly like standard bindings and split the view.
I think something bad is going on with the tabs inside tabs thing in the new Xcode that is messing with things. Maybe this relates back to me not being updated to Big Sur, time will tell.
Please report your findings, this is killing me :)
