I have Sketchup Pro 23.1.240. My Default Tray doesn’t appear when the mouse pointer goes to the righthand of the screen. I’m just curious, because I’ve found a better method by make the Tray appear a shortcut. I never like the old method anyway.
If you use the little pin to Auto Hide it it will open whenever you hover over there, but if you close it you have to reopen it from the window menu.
If you do have to reload it from the menu then it might not dock when you drag it to the side. The correct process is to drag it to the middle of the window and then you will get a selector for which side or top / bottom edge you want to dock it to.
Stumped me for a while till someone here helped me out with it.
Alternatively double clicking on the tray’s title bar will dock it in its default location.
That didn’t actually work for me. At least it didn’t dock just overlaid when I tried that.
I am not sure when the drag to the middle function was introduced. Could have been a long time since my Tray was undocked. Anyway it was new to me.
It is a pity that the UX that is common at least in the Windows world, where dragging a window to an edge docks it, could not be used in SU as that is intuitive for most users now.
Hmmm… It clearly worked for me. I wonder why not for you.
It’s been that way since the tray thing was first introduced in SU 2016.
Told you it has been a long time 
It’s worth noting that the docking tabs can be found at the edges as well as the middle.
Ok I don’t understand why but my install ONLY has tabs in the middle of the screen.
BUT as per Dave’s advice double clicking on the header does take the panel back to its last position. I think the problem is if it has “lost” its position that doesn’t work as it doesnt’ know where to return to. You can test this by docking the panel to the other side, dragging it away and double clicking and it returns to its most recent position.
Anyway now just an item of mild interest receding in the rear view mirror 