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

[Eli Zaretskii <eliz@gnu.org>]

> 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:

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)))))