From aaef7a6438aaa74a5978e5e9d4b5e95910c2b15e Mon Sep 17 00:00:00 2001
From: Augusto Stoffel <arstoffel@gmail.com>
Date: Wed, 27 Jan 2021 16:09:38 +0100
Subject: [PATCH] Reduce flicker in Isearch mode

Lazy highlighting now happens immediately (only) if the search string
is longer than the value of the new custom variable
`lazy-highlight-delay-max-chars`.  Also avoid updating the lazy count
in the echo area too often.
* isearch.el (lazy-highlight-delay-max-chars): new defcustom
* isearch.el (isearch-lazy-highlight-with-timer): new function,
factors out a repeated snippet of code.
* isearch.el (isearch-lazy-highlight-new-loop,
isearch-lazy-highlight-update, isearch-lazy-highlight-buffer-update):
use the new `isearch-lazy-highlight-with-timer` function.
* isearch.el (isearch-lazy-highlight-new-loop): avoid a call to
`isearch-message` that is quickly succeed by a second echo area
update, thus causing flicker.
---
 lisp/isearch.el | 48 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 16 deletions(-)

diff --git a/lisp/isearch.el b/lisp/isearch.el
index a86678572c..ca0fd1434a 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -352,12 +352,22 @@ lazy-highlight-cleanup
   :group 'lazy-highlight)
 
 (defcustom lazy-highlight-initial-delay 0.25
-  "Seconds to wait before beginning to lazily highlight all matches."
+  "Seconds to wait before beginning to lazily highlight all matches.
+This setting only has effect if the current search string is at most
+`lazy-highlight-initial-delay' character long."
   :type 'number
   :group 'lazy-highlight)
 
+(defcustom lazy-highlight-delay-max-chars 2
+  "Size of a search string above which lazy highlight happens immediately."
+  :type 'number
+  :group 'lazy-highlight
+  :version "28.1")
+
 (defcustom lazy-highlight-interval 0 ; 0.0625
-  "Seconds between lazily highlighting successive matches."
+  "Seconds between lazily highlighting successive matches.
+This setting only has effect if the current search string is at most
+`lazy-highlight-initial-delay' character long."
   :type 'number
   :group 'lazy-highlight)
 
@@ -3357,7 +3367,7 @@ isearch-lazy-count-format
              (not isearch-error)
              (not isearch-suspended))
         (format format-string
-                (if isearch-forward
+                (if isearch-lazy-highlight-forward
                     isearch-lazy-count-current
                   (if (eq isearch-lazy-count-current 0)
                       0
@@ -3858,6 +3868,15 @@ lazy-highlight-cleanup
     (cancel-timer isearch-lazy-highlight-timer)
     (setq isearch-lazy-highlight-timer nil)))
 
+(defun isearch-lazy-highlight-with-timer (delay function)
+  "Arrange for FUNCTION to be called in idle time, possibly with DELAY.
+There is no delay if the current search string is longer than
+`lazy-highlight-delay-max-chars'."
+  (setq isearch-lazy-highlight-timer
+        (run-with-idle-timer
+         (if (> (length isearch-string) lazy-highlight-delay-max-chars) 0 delay)
+         nil function)))
+
 (defun isearch-lazy-highlight-new-loop (&optional beg end)
   "Cleanup any previous `lazy-highlight' loop and begin a new one.
 BEG and END specify the bounds within which highlighting should occur.
@@ -3917,7 +3936,8 @@ isearch-lazy-highlight-new-loop
         (clrhash isearch-lazy-count-hash)
         (setq isearch-lazy-count-current nil
               isearch-lazy-count-total nil)
-        (isearch-message)))
+        ;; Delay updating the message if possible, to avoid flicker
+        (when (string-equal isearch-string "") (isearch-message))))
     (setq isearch-lazy-highlight-window-start-changed nil)
     (setq isearch-lazy-highlight-window-end-changed nil)
     (setq isearch-lazy-highlight-error isearch-error)
@@ -3961,9 +3981,8 @@ isearch-lazy-highlight-new-loop
 		    (1- (length isearch-lazy-highlight-last-string)))
 		 (point-min))))
     (unless (equal isearch-string "")
-      (setq isearch-lazy-highlight-timer
-            (run-with-idle-timer lazy-highlight-initial-delay nil
-                                 'isearch-lazy-highlight-start))))
+      (isearch-lazy-highlight-with-timer lazy-highlight-initial-delay
+                                         'isearch-lazy-highlight-start)))
   ;; Update the current match number only in isearch-mode and
   ;; unless isearch-mode is used specially with isearch-message-function
   (when (and isearch-lazy-count isearch-mode (null isearch-message-function))
@@ -4101,12 +4120,10 @@ isearch-lazy-highlight-update
 		  (if isearch-lazy-highlight-forward
 		      (setq isearch-lazy-highlight-end (point-min))
 		    (setq isearch-lazy-highlight-start (point-max)))
-		  (setq isearch-lazy-highlight-timer
-			(run-at-time lazy-highlight-interval nil
-				     'isearch-lazy-highlight-buffer-update)))
-	      (setq isearch-lazy-highlight-timer
-		    (run-at-time lazy-highlight-interval nil
-				 'isearch-lazy-highlight-update)))))))))
+	          (isearch-lazy-highlight-with-timer
+	           lazy-highlight-interval 'isearch-lazy-highlight-buffer-update))
+	      (isearch-lazy-highlight-with-timer
+               lazy-highlight-interval 'isearch-lazy-highlight-update))))))))
 
 (defun isearch-lazy-highlight-buffer-update ()
   "Update highlighting of other matches in the full buffer."
@@ -4180,9 +4197,8 @@ isearch-lazy-highlight-buffer-update
 		  (setq isearch-lazy-count-current
 			(gethash opoint isearch-lazy-count-hash 0))
 		  (isearch-message))
-	      (setq isearch-lazy-highlight-timer
-		    (run-at-time lazy-highlight-interval nil
-				 'isearch-lazy-highlight-buffer-update)))))))))
+	      (isearch-lazy-highlight-with-timer
+	       lazy-highlight-interval 'isearch-lazy-highlight-buffer-update))))))))
 
 (defun isearch-resume (string regexp word forward message case-fold)
   "Resume an incremental search.
-- 
2.29.2