On my AT3, if I stop while in first gear, the gear lever is solid. I cannot select neutral or second gear. I have to pull away to get out of first. Rocking does not work. While I am moving, it all works fine. The clutch is not dragging when I am stopped and in gear so I don't think it's that.

What suggestions do you have?

Thanks

The eccentric stopper pin that controls the shift shaft end points may need adjusting.

Is that a crankcase split or is it accessed behind a side cover?

It would be worth checking the neutral detent plunger, which screws into the cases, just above the gearbox sprocket. It has a 14mm hex head. Obviously, you have already checked the gearbox oil?

Unusual thing to happen but had something similar with an RX125 40 years ago where for some reason it was pulling the drum too far & the claw arm pointy bit was trying to return under the drum pin & jammed.
Don't think it was first gear though but all gears should have the same problem as equal movement between gears.
By memory it was to do with the stop screw bent or way out of adjustment or had had a good jolt from an accident & was lose in the thread so got taken slightly further up & down due to wobble Down being the main problem because the pointy end only comes into play on down changes..
Not sure that's your problem but the screw is a good place to start [as Tinkicker suggests].
Rethinking, if you hook it up into third or 4th gear, the free springy movement of the gear lever should be close to equal before it touches the next pin to change up or down a gear.
Check that before you go in there. If it feels normal then--i'm lost for now.
Driving me mad trying to edit as loaded the wording three time when i added pics. I should know how to do it by now..

Behind clutch cover.  Number 4 on piccy.

Did this just start happening from a rebuild ?

You specifically said the lever is "solid".  That's different than being able to lift up on the lever and it just doesn't do anything. 

If lever truly is solid, then it's doubtful to be an adjustment? 

What happens if you shut off engine?  Are you still stuck in first gear?

The clutch FIRST needs to be re-adjusted.

loosen the clutch lever at the perch.
adjust the actual push pin - back it off slightly.
then go back to the clutch lever and give yourself minimal free play.

you’ve experienced cable stretching.

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As it only takes minutes to do, removing the neutral detent plunger, from the top of the cases, and seeing if that makes any difference might be a good idea?

The other thing that is worth checking, is whether the chain coming off, or breaking, has damaged the part of the crankcases, where the left-hand end of the shift drum fits.