Try using flushSync when flushing Reagent render queue

This will avoid double queuing when Reagent has already queued
components to render due to ratom changes.

Author: Deraen

src/reagent/dom/client.cljs

@@ -1,6 +1,7 @@
 (ns reagent.dom.client
   (:require ["react" :as react]
             ["react-dom/client" :as react-dom-client]
+            ["react-dom" :as react-dom]
             [reagent.impl.batching :as batch]
             [reagent.impl.protocols :as p]
             [reagent.impl.template :as tmpl]
@@ -52,6 +53,7 @@
   ([^js root el]
    (render root el tmpl/*current-default-compiler*))
   ([^js root el compiler]
+   (set! batch/react-flush react-dom/flushSync)
    (let [comp (fn [] (p/as-element compiler el))
          js-props #js {}]
      (set! (.-comp js-props) comp)
@@ -62,6 +64,7 @@
    (hydrate-root container el nil))
   ([container el {:keys [compiler on-recoverable-error identifier-prefix]
                   :or {compiler tmpl/*current-default-compiler*}}]
+   (set! batch/react-flush react-dom/flushSync)
    (let [js-props #js {}
          comp (fn [] (p/as-element compiler el))]
      (set! (.-comp js-props) comp)

src/reagent/impl/batching.cljs

@@ -29,14 +29,27 @@
   (- (.-cljsMountOrder c1)
      (.-cljsMountOrder c2)))
 
+;; See reagent.dom.client/render and hydrate-root
+;;
+;; On React 19 react-dom/flushSync is used to flush all Reagent ratom changes
+;; to DOM right away, to avoid double batching for these.
+;; React strongly recommends to avoid flushSync, but run-queue is
+;; used just once per animation frame and only when there were component
+;; re-renders triggered from ratom changes, so maybe this is fine.
+(defonce react-flush
+  (fn noop [f]
+    (f)))
+
 (defn run-queue [a]
   ;; sort components by mount order, to make sure parents
   ;; are rendered before children
   (.sort a compare-mount-order)
-  (dotimes [i (alength a)]
-    (let [^js/React.Component c (aget a i)]
-      (when (true? (.-cljsIsDirty c))
-        (.forceUpdate c)))))
+  (react-flush
+    (fn []
+      (dotimes [i (alength a)]
+        (let [^js/React.Component c (aget a i)]
+          (when (true? (.-cljsIsDirty c))
+            (.forceUpdate c)))))))
 
 
 ;; Set from ratom.cljs