Reduce flicker in Isearch mode

Lazy highlighting now happens immediately when the search string is at
least as long as the value of the new custom variable
`lazy-highlight-no-delay-length`.  Also avoid updating the lazy
count in the echo area too often.
* isearch.el (lazy-highlight-no-delay-length): New defcustom.
* isearch.el (lazy-lazy-count-format): Avoid a momentarily incorrect
count when reversing search direction.
* 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.
* isearch.el (isearch-lazy-highlight-new-loop):
Start lazy highlight immediately if appropriate.
* etc/NEWS: Announce the change.
* doc/emacs/search.texi: Document `lazy-highlight-no-delay-length'.

Copyright-paperwork-exempt: yes

3 files changed, 33 insertions, 4 deletions

diff --git a/doc/emacs/search.texi b/doc/emacs/search.texi
index 637867b8115..f3c42bcea7f 100644
--- a/doc/emacs/search.texi
+++ b/doc/emacs/search.texi
@@ -2027,6 +2027,13 @@ highlighting:
 @item lazy-highlight-initial-delay
 @vindex lazy-highlight-initial-delay
 Time in seconds to wait before highlighting visible matches.
+Applies only if the search string is less than
+@code{lazy-highlight-no-delay-length} characters long.
+
+@item lazy-highlight-no-delay-length
+@vindex lazy-highlight-no-delay-length
+For search strings at least as long as the value of this variable,
+lazy highlighting of matches starts immediately.
 
 @item lazy-highlight-interval
 @vindex lazy-highlight-interval
diff --git a/etc/NEWS b/etc/NEWS
index 7b4b7fea5a2..483375e8a2e 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -226,6 +226,13 @@ C-M-<return> instead of <C-M-return>.  Either variant can be used as
 input; functions such as 'kbd' and 'read-kbd-macro' accept both styles
 as equivalent (they have done so for a long time).
 
++++
+** New user option 'lazy-highlight-no-delay-length'.
+Lazy highlighting of matches in Isearch now starts immediately if the
+search string is at least this long.  'lazy-highlight-initial-delay'
+still applies for shorter search strings, which avoids flicker in the
+search buffer due to too many matches being highlighted.
+
 
 * Editing Changes in Emacs 28.1
 
diff --git a/lisp/isearch.el b/lisp/isearch.el
index a1e3fe2c3f0..a6978a4c164 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -352,10 +352,20 @@ If this is nil, extra highlighting can be \"manually\" removed with
   :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 when the search string is less than
+`lazy-highlight-no-delay-length' characters long."
   :type 'number
   :group 'lazy-highlight)
 
+(defcustom lazy-highlight-no-delay-length 3
+  "For search strings at least this long, lazy highlight starts immediately.
+For shorter search strings, `lazy-highlight-initial-delay'
+applies."
+  :type 'number
+  :group 'lazy-highlight
+  :version "28.1")
+
 (defcustom lazy-highlight-interval 0 ; 0.0625
   "Seconds between lazily highlighting successive matches."
   :type 'number
@@ -3356,7 +3366,7 @@ isearch-message-suffix prompt.  Otherwise, for isearch-message-prefix."
              (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
@@ -3916,7 +3926,8 @@ by other Emacs features."
         (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)
@@ -3961,7 +3972,11 @@ by other Emacs features."
 		 (point-min))))
     (unless (equal isearch-string "")
       (setq isearch-lazy-highlight-timer
-            (run-with-idle-timer lazy-highlight-initial-delay nil
+            (run-with-idle-timer (if (>= (length isearch-string)
+                                         lazy-highlight-no-delay-length)
+                                     0
+                                   lazy-highlight-initial-delay)
+                                 nil
                                  'isearch-lazy-highlight-start))))
   ;; Update the current match number only in isearch-mode and
   ;; unless isearch-mode is used specially with isearch-message-function