Failed Homing
Unboxed, assembled display, ran checks, all ok and ready to print.
Repeatedly whacked into the stops on X and Y axes for what seemed like minutes, eventually failed homing. Reset, same problem. Power off, unplug, try again, same.
Very disappointed with Prusa at that point.
The wisdom of the internet suggested adding tension to the belts so I gave them one turn. It worked!
The next day I backed off 1/2 turn and it worked so I printed the Benchy, looks ok.
I removed the top cover and checked the gaps on the left and right sides of the gantry: at Y = -17 the gaps were L = 1.5 mm, R = 1.25 mm. Seems good to me. The belts are obviously different tension (finger pluck test). I will try a bigger print and see how it goes.
RE: Failed Homing
There's a long thread about this problem: https://forum.prusa3d.com/forum/postid/737002/
You don't want the belt tension to be unequal. Both belts should be close to the nominal 85 Hz frequency in the "pluck test"; use the tuner in the Prusa App to check. If the X gantry is not sitting straight with those belt tensions, the latest advice from Prusa support is to try and bend the steel brackets which support the gantry -- see the last two pages or so of the thread.
Not sure I like that advice, since it will put stress on the linear bearings for the Y axis. The Prusa technicians should have assembled it properly in the first place...