bug#3357: 23.0.93; Loss of undo info in calc

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

1. M-x calc
2. do some calculation for example: ' sqrt ( x ) RET a t x 5 RET
3. type q to quit calc
4. M-x calc to bring it back
5. type U

You will get "No further undo information is available".

I think this is a major inconvenience. For example, let's say in the
above example I subsitute x with some numerical value to get a result. I
think I have got what I want so I quit the window. But quite often I
need another result at a different x. When I bring back calc, I won't be
able to go back to the taylor series expression and have to start over.
The situation is even worse with curve fitting where you have to put in
the data points first.

Incidentally, I wonder whether making the two windows dedicated is more
desirable than current behaviour. I often end up with for example the
*Help* buffer showing up in the tiny "*Calc Trail*" window.

Thanks.

Leo