Scroll half of the visible screen respecting zoom level (#1490)

* evil-scroll half of the visible screen, respecting zoom level

* remove unnecessary interactive declaration

Author: countvajhula

evil-commands.el

@@ -1043,7 +1043,7 @@ If the scroll count is zero the command scrolls half the screen."
     (when (= (point-min) (line-beginning-position))
       (signal 'beginning-of-buffer nil))
     (when (zerop count)
-      (setq count (/ (window-body-height) 2)))
+      (setq count (/ (evil-window-visible-height) 2)))
     (let ((xy (evil-posn-x-y (posn-at-point))))
       (condition-case nil
           (progn
@@ -1067,7 +1067,7 @@ If the scroll count is zero the command scrolls half the screen."
     (setq evil-scroll-count count)
     (when (eobp) (signal 'end-of-buffer nil))
     (when (zerop count)
-      (setq count (/ (window-body-height) 2)))
+      (setq count (/ (evil-window-visible-height) 2)))
     ;; BUG #660: First check whether the eob is visible.
     ;; In that case we do not scroll but merely move point.
     (if (<= (point-max) (window-end))

evil-common.el

@@ -3980,6 +3980,21 @@ PROPERTIES is a property-list which supports the following properties:
          (evil-motion-state))
        (switch-to-buffer-other-window buf))))
 
+;;; Window
+
+(defun evil-window-visible-height (&optional window)
+  "The visible height of WINDOW in lines.
+
+If no WINDOW is specified, use the selected one."
+  (let ((window (or window (selected-window))))
+    (save-window-excursion
+      (select-window window)
+      (let ((window-top (save-excursion (move-to-window-line 0)
+                                        (line-number-at-pos)))
+            (window-bottom (save-excursion (move-to-window-line -1)
+                                           (line-number-at-pos))))
+        (- window-bottom window-top)))))
+
 (provide 'evil-common)
 
 ;;; evil-common.el ends here