Make pixel-wise scrolling less laggy

* lisp/pixel-scroll.el (pixel-dead-time, pixel-last-scroll-time):
New variables.
(pixel-scroll-up, pixel-scroll-down): Invoke 'scroll-up' or
'scroll-down' when called within 'pixel-dead-time'.  (Bug#29737)

lisp/pixel-scroll.el

@@ -82,6 +82,27 @@ case you need scrolling resolution of a pixel, set to 1.  After a
 pixel scroll, typing \\[next-line] or \\[previous-line] scrolls the window to make it
 fully visible, and undoes the effect of the pixel-level scroll.")
 
+(defvar pixel-dead-time 0.1
+  "Minimal interval in seconds before next smooth scrolling.
+If another scrolling request arrives within this period, scrolling
+will be carried out without pixel resolution.  If zero, scrolling
+is always with pixel resolution.")
+
+(defvar pixel-last-scroll-time 0
+  "Time when the last scrolling was made, in second since the epoch.")
+
+(defun pixel-scroll-in-rush-p ()
+  "Return non-nil if next scroll should be non-smooth.
+When scrolling request is delivered soon after the previous one,
+user is in hurry.  When the time since last scroll is larger than
+`pixel-dead-time', we are ready for another smooth scroll, and this
+function returns nil."
+  (let* ((current-time (float-time))
+         (scroll-in-rush-p (< (- current-time pixel-last-scroll-time)
+                              pixel-dead-time)))
+    (setq pixel-last-scroll-time current-time)
+    scroll-in-rush-p))
+
 ;;;###autoload
 (define-minor-mode pixel-scroll-mode
   "A minor mode to scroll text pixel-by-pixel.
@@ -104,6 +125,8 @@ if ARG is omitted or nil."
 This is an alternative of `scroll-up'.  Scope moves downward."
   (interactive)
   (or arg (setq arg 1))
+  (if (pixel-scroll-in-rush-p)
+      (scroll-up arg)
     (dotimes (ii arg)                    ; move scope downward
       (let ((amt (if pixel-resolution-fine-flag
                      (if (integerp pixel-resolution-fine-flag)
@@ -114,13 +137,15 @@ This is an alternative of `scroll-up'.  Scope moves downward."
             (scroll-up 1)             ; relay on robust method
           (while (pixel-point-at-top-p amt) ; prevent too late (multi tries)
             (vertical-motion 1))            ; move point downward
-        (pixel-scroll-pixel-up amt)))))   ; move scope downward
+          (pixel-scroll-pixel-up amt))))))  ; move scope downward
 
 (defun pixel-scroll-down (&optional arg)
   "Scroll text of selected window down ARG lines.
 This is and alternative of `scroll-down'.  Scope moves upward."
   (interactive)
   (or arg (setq arg 1))
+  (if (pixel-scroll-in-rush-p)
+      (scroll-down arg)
     (dotimes (ii arg)
       (let ((amt (if pixel-resolution-fine-flag
                      (if (integerp pixel-resolution-fine-flag)
@@ -132,7 +157,7 @@ This is and alternative of `scroll-down'.  Scope moves upward."
         (if (or (pixel-bob-at-top-p amt) ; when beginning-of-the-buffer is seen
                 (pixel-eob-at-top-p))    ; for file with a long line
             (scroll-down 1)              ; relay on robust method
-        (pixel-scroll-pixel-down amt)))))
+          (pixel-scroll-pixel-down amt))))))
 
 (defun pixel-bob-at-top-p (amt)
   "Return non-nil if window-start is at beginning of the current buffer.