bug#54800: 28.1; calc: Cursor positioned at weird position in calc stack

[Christoph Arenz <tiga.arenz@web.de>]

On 09.04.22 09:34, Eli Zaretskii wrote:
>> Date: Fri, 8 Apr 2022 21:44:38 +0200
>> From: Christoph Arenz <tiga.arenz@web.de>
>>
>> When the content of the calc stack inclusive top-of-stack symbol `.'
>> gets larger in height than the calc window, the cursor is positioned at
>> a weird position in the upper half of the stack window. When
>> line-numbering is on, the cursor is positioned at the beginning of the
>> line at the line number. Calling `calc-realign' does not change the
>> cursor postion.
>>
>> How to reproduce:
>> 1. emacs -Q
>> 2. C-x * * (to start calc)
>> 3. enter a number and RET
>> 4. repeat No. 3 until the stacks grows and the symptom appears
>>
>> Expected behavior for the cursor is to be placed on the `.' symbol,
>> representing the top of the stack.
>>
>> 5. After pressing `o' [calc-realign], the expected behavior would also
>> be for the cursor to be positioned at the top of the calc stack.
>>
>> Debugging the function calc-align-stack-window with edebug, the symptom
>> goes away when placing a breakpoint at the line
>> (calc-cursor-stack-index 0) just after the set-window-start call, and
>> then continuing from there.
>>
>> The following patch fixes the problem for me, but I do not fully
>> understand the NOFORCE option of the set-window-start function.
> Thanks, but I think using NOFORCE is not the best solution here,
> because that causes Emacs to recenter the stack window, thus showing
> fewer stack entries than possible.
>
> I think the bug is actually off-by-one error, because the
> vertical-motion call doesn't take the header-line into account.  So I
> suggest the following patch instead, please see if it solves the
> problem:

Your patch solves the problem and shows maximum content of the stack
in the window.
Note that it still contains the NOFORCE while the off-by-one fix also
works without it.

> diff --git a/lisp/calc/calc.el b/lisp/calc/calc.el
> index 81677d7..900940d 100644
> --- a/lisp/calc/calc.el
> +++ b/lisp/calc/calc.el
> @@ -1815,8 +1815,8 @@ calc-align-stack-window
>   	  (if win
>   	      (progn
>   		(calc-cursor-stack-index 0)
> -		(vertical-motion (- 2 (window-height win)))
> -		(set-window-start win (point)))))
> +		(vertical-motion (- 3 (window-height win)))
> +		(set-window-start win (point) 'noforce))))
>   	(calc-cursor-stack-index 0)
>   	(if (looking-at " *\\.$")
>   	    (goto-char (1- (match-end 0)))))