Fix term.el cursor movement at bottom margin (Bug#31690)

* lisp/term.el (term-handle-ansi-escape) <\E[B cud>: Allow moving the
cursor to the bottom margin line, rather than stopping one line
before.

lisp/term.el

@@ -3387,11 +3387,10 @@ option is enabled.  See `term-set-goto-process-mark'."
    ;; \E[B - cursor down (terminfo: cud)
    ((eq char ?B)
     (let ((tcr (term-current-row)))
-      (unless (= tcr (1- term-scroll-end))
+      (unless (>= tcr term-scroll-end)
 	(term-down
-	 (if (> (+ tcr term-terminal-parameter) term-scroll-end)
-	     (- term-scroll-end 1 tcr)
-	   (max 1 term-terminal-parameter)) t))))
+         (min (- term-scroll-end tcr) (max 1 term-terminal-parameter))
+         t))))
    ;; \E[C - cursor right (terminfo: cuf, cuf1)
    ((eq char ?C)
     (term-move-columns

test/lisp/term-tests.el

@@ -124,6 +124,27 @@ line6\r
                     40 12 (list "\eAnSiTc /f" "oo/\n") 'default-directory)
                    "/foo/"))))
 
+(ert-deftest term-to-margin ()
+  "Test cursor movement at the scroll margin.
+This is a reduced example from GNU nano's initial screen."
+  (let* ((width 10)
+         (x (make-string width ?x))
+         (y (make-string width ?y)))
+    (should (equal (term-test-screen-from-input
+                    width 3
+                    `("\e[1;3r"       ; Setup 3 line scrolling region.
+                      "\e[2;1H"       ; Move to 2nd last line.
+                      ,x              ; Fill with 'x'.
+                      "\r\e[1B"       ; Next line.
+                      ,y))            ; Fill with 'y'.
+                   (concat "\n" x "\n" y)))
+    ;; Same idea, but moving upwards.
+    (should (equal (term-test-screen-from-input
+                    width 3
+                    `("\e[1;3r" "\e[2;1H" ,x "\r\e[1A" ,y))
+                   (concat y "\n" x)))))
+
+
 (provide 'term-tests)
 
 ;;; term-tests.el ends here