Improve behavior entering fragments from the end

If a LaTeX environment is disabled by the point entering from the end of
the fragment, then the point is moved to the last line of the fragment
and the column of the point is preserved. Previously, the point always
moved to the start of the LaTeX environment.

Co-authored-by: Benjamin Levy <[email protected]>

org-fragtog.el

@@ -195,7 +195,8 @@ return nil."
   ;; The fragment must be disabled before `org-latex-preview', since
   ;; `org-latex-preview' only toggles, leaving no guarantee that it's enabled
   ;; afterwards.
-  (org-fragtog--disable-frag frag)
+  (save-excursion
+    (org-fragtog--disable-frag frag))
 
   ;; Move to fragment and enable
   (save-excursion
@@ -210,6 +211,27 @@ return nil."
     (re-search-forward "[^ \t]")
     (ignore-errors (org-latex-preview))))
 
+(defun org-fragtog--set-point-after-disable-frag (frag)
+  "Set point to where it should be after FRAG was disabled.
+If point decreases and enters a fragment from the end, disabling it, then point
+should move to the end of the fragment.  If point moved up one line, its column
+should be maintained."
+  (when (and ;; There has to be a prev-point.
+         org-fragtog--prev-point
+         ;; Only move to the end of the fragment if it's closer to the
+         ;; prev-point location than the start of the fragment is.
+         (< (abs (- org-fragtog--prev-point (org-fragtog--frag-end frag)))
+            (abs (- org-fragtog--prev-point (org-fragtog--frag-start frag)))))
+    (let ((prev-point-column (save-excursion
+                               (goto-char org-fragtog--prev-point)
+                               (current-column))))
+      ;; Move to the line where the fragment ends while preserving the point
+      ;; column.
+      (goto-char (1- (org-fragtog--frag-end frag)))
+      (when (/= (line-number-at-pos org-fragtog--prev-point)
+                (line-number-at-pos))
+        (move-to-column prev-point-column)))))
+
 (defun org-fragtog--disable-frag (frag &optional renew)
   "Disable the Org LaTeX fragment preview for the fragment FRAG.
 If RENEW is non-nil, renew the fragment at point."
@@ -224,7 +246,8 @@ If RENEW is non-nil, renew the fragment at point."
   ;; There may be nothing at the adjusted point
   (when frag
     (org-clear-latex-preview (org-fragtog--frag-start frag)
-                             (org-fragtog--frag-end frag))))
+                             (org-fragtog--frag-end frag))
+    (org-fragtog--set-point-after-disable-frag frag)))
 
 (provide 'org-fragtog)