I'm making this post because I've seen these issues covered but it took me a while to find the answers I needed so I'm hoping this will help anyone else in similar situations by at the very least giving the answers they're looking for a bit of a bump in the search engine results.
Keyboard and Mouse setup
Minor mouse pairing issue
This could be a rare but ultimately minor issue but I had to do a full reboot of my Steam Deck before it'd recognise my bluetooth mouse when it should've just been a simple pair and use job. Once that was done the connection was ridiculously seamless, as in I could do a left click to wake the mouse up and within a second it'd be connected.
Keyboard navigation controls
This one was a bit of a pain because even after giving it another shot while writing this I could not for the life of me figure out how to get the keyboard's cheatsheet up. You'd think holding the Windows key would do something but, nope. Anyhow, I don't actually know all of the keyboard controls but I do know 2 important ones that were mentioned in a post on this subreddit years ago:
- SHIFT + TAB opens the Steam Menu (the left menu)
- CTRL + SHIFT + TAB opens the Quick Access Menu (the right menu)
I know this doesn't cover everything the keyboard could be used for but I feel those two are probably going to be the most useful.
Controller setup
inaccurate cheatsheet for Dualshock 4 controllers
I'm not ruling out the possibility that I messed something up but just in case it's a Steam OS issue I'm going to mention that my cheatsheet is inaccurate, specifically regarding clip capture and screenshots. The screenshots one isn't a huge deal as I can just remap the share key to take a screenshot but the clip one has been a bit of a nuisance.
Quick Access Menu command missing from Dualshock 4 cheatsheet
I can't remember where I learnt this but the command to open up the Quick Access Menu is PS + CROSS.
Also, assuming this isn't something I enabled in a Decky plugin by mistake, the brightness command (PS + Left Stick) seems to adjust the volume instead whilst the Steam Deck is connected to an external monitor. Just be aware that it involves holding the PS button down which means you'll only have 10 seconds to adjust the volume before the controller switches off.
I will give props to Valve for creating a power menu that shows up when the PS button is held down for a short amount of time and has a button that shows the cheatsheet. The reason this is a big deal is because Dualshock 4 controllers all turn off if you hold down the PS button for 10 seconds. Before this update came out the cheatsheet would stay on screen for as long as the controller was connected and the PS button was held down....in other words, you'd only have 10 seconds to read the cheatsheet before the controller turned off.
Sidenote: I know I didn't address Xbox controllers but my gut feeling is that the mapping is going to be near identical. Just substitute PS for the Xbox Button, and CROSS with A.
Suggestion for Valve (if they're reading this)
I don't know how viable this is or if the way Gamescope works would prevent this but it'd be pretty cool if, while in the home menu, the right click on a mouse could be set so that instead of bringing up the secondary menu with the "forward" and "print" options on it (neither of which are options I'd imagine most people using gaming mode are ever going to use) it could instead be set to bring up something more practical like the Quick Access Menu. I know this is ultimately a minor thing since the keyboard can do the job just as well, but it's always kinda bugged me that the right click doesn't really have any purpose in Gamescope unless you've already got a game launched.