Fix ruler-mode text-scaling issues

* lisp/ruler-mode.el (ruler-mode-text-scaled-width): New function
for computing scaled text width.
(ruler-mode-text-scaled-window-hscroll)
(ruler-mode-text-scaled-window-width): Compute text scaled
`window-width' value.
(ruler-mode-mouse-grab-any-column, ruler-mode-mouse-add-tab-stop)
(ruler-mode-ruler): Change `window-hscroll' into
`ruler-mode-text-scaled-window-hscroll', and change `window-width'
into `ruler-mode-text-scaled-window-width'.

lisp/ruler-mode.el

@@ -360,6 +360,20 @@ START-EVENT is the mouse click event."
 That is `fill-column', `comment-column', `goal-column', or nil when
 nothing is dragged.")
 
+(defun ruler-mode-text-scaled-width (width)
+  "Compute scaled text width according to current font scaling.
+Convert a width of char units into a text-scaled char width units,
+Ex. `window-hscroll'."
+  (/ (* width (frame-char-width)) (default-font-width)))
+
+(defun ruler-mode-text-scaled-window-hscroll ()
+  "Text scaled `window-hscroll'."
+  (ruler-mode-text-scaled-width (window-hscroll)))
+
+(defun ruler-mode-text-scaled-window-width ()
+  "Text scaled `window-width'."
+  (ruler-mode-text-scaled-width (window-width)))
+
 (defun ruler-mode-mouse-grab-any-column (start-event)
   "Drag a column symbol on the ruler.
 Start dragging on mouse down event START-EVENT, and update the column
@@ -372,9 +386,9 @@ dragging.  See also the variable `ruler-mode-dragged-symbol'."
     (save-selected-window
       (select-window (posn-window start))
       (setq col  (ruler-mode-window-col (car (posn-col-row start)))
-            newc (+ col (window-hscroll)))
+            newc (+ col (ruler-mode-text-scaled-window-hscroll)))
       (and
-       (>= col 0) (< col (window-width))
+       (>= col 0) (< col (ruler-mode-text-scaled-window-width))
        (cond
 
         ;; Handle the fill column.
@@ -457,8 +471,8 @@ Called on each mouse motion event START-EVENT."
     (save-selected-window
       (select-window (posn-window start))
       (setq col  (ruler-mode-window-col (car (posn-col-row end)))
-            newc (+ col (window-hscroll)))
-      (when (and (>= col 0) (< col (window-width)))
+            newc (+ col (ruler-mode-text-scaled-window-hscroll)))
+      (when (and (>= col 0) (< col (ruler-mode-text-scaled-window-width)))
         (set ruler-mode-dragged-symbol newc)))))
 
 (defun ruler-mode-mouse-add-tab-stop (start-event)
@@ -473,8 +487,8 @@ START-EVENT is the mouse click event."
         (save-selected-window
           (select-window (posn-window start))
           (setq col (ruler-mode-window-col (car (posn-col-row start)))
-                ts  (+ col (window-hscroll)))
-          (and (>= col 0) (< col (window-width))
+                ts  (+ col (ruler-mode-text-scaled-window-hscroll)))
+          (and (>= col 0) (< col (ruler-mode-text-scaled-window-width))
                (not (member ts tab-stop-list))
                (progn
                  (message "Tab stop set to %d" ts)
@@ -494,8 +508,8 @@ START-EVENT is the mouse click event."
         (save-selected-window
           (select-window (posn-window start))
           (setq col (ruler-mode-window-col (car (posn-col-row start)))
-                ts  (+ col (window-hscroll)))
-          (and (>= col 0) (< col (window-width))
+                ts  (+ col (ruler-mode-text-scaled-window-hscroll)))
+          (and (>= col 0) (< col (ruler-mode-text-scaled-window-width))
                (member ts tab-stop-list)
                (progn
                  (message "Tab stop at %d deleted" ts)
@@ -648,11 +662,11 @@ Optional argument PROPS specifies other text properties to apply."
 
 (defun ruler-mode-ruler ()
   "Compute and return a header line ruler."
-  (let* ((w (window-width))
+  (let* ((w (ruler-mode-text-scaled-window-width))
          (m (window-margins))
          (f (window-fringes))
          (i 0)
-         (j (window-hscroll))
+         (j (ruler-mode-text-scaled-window-hscroll))
          ;; Setup the scrollbar, fringes, and margins areas.
          (lf (ruler-mode-space
               'left-fringe