Isomorphic hydration: skip re-render when server HTML present

sx-mount now checks if the target element has children (server-
rendered HTML). If so, skips the client re-render and only runs
hydration (process-elements, hydrate-islands, hydrate-elements).

This preserves server-rendered CSSX styling and avoids the flash
of unstyled content that occurred when the client replaced the
server HTML before re-rendering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-23T15:39:32Z";
+  var SX_VERSION = "2026-03-23T16:18:55Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }
@@ -4627,16 +4627,12 @@ PRIMITIVES["hoist-head-elements-full"] = hoistHeadElementsFull;
   // sx-mount
   var sxMount = function(target, source, extraEnv) { return (function() {
   var el = resolveMountTarget(target);
-  return (isSxTruthy(el) ? (function() {
+  return (isSxTruthy(el) ? ((isSxTruthy(isEmpty(domChildList(el))) ? (function() {
   var node = sxRenderWithEnv(source, extraEnv);
   domSetTextContent(el, "");
   domAppend(el, node);
-  hoistHeadElementsFull(el);
+  return hoistHeadElementsFull(el);
-  processElements(el);
+})() : NIL), processElements(el), sxHydrateElements(el), sxHydrateIslands(el), runPostRenderHooks()) : NIL);
-  sxHydrateElements(el);
-  sxHydrateIslands(el);
-  return runPostRenderHooks();
-})() : NIL);
 })(); };
 PRIMITIVES["sx-mount"] = sxMount;
 

web/boot.sx

@@ -79,16 +79,19 @@
     ;; extra-env: optional extra bindings dict
     (let ((el (resolve-mount-target target)))
       (when el
-        (let ((node (sx-render-with-env source extra-env)))
+        ;; If the server already rendered content (isomorphic SSR),
-          (dom-set-text-content el "")
+        ;; skip re-render — just hydrate the existing DOM.
-          (dom-append el node)
+        (when (empty? (dom-child-list el))
-          ;; Hoist head elements from rendered content
+          (let ((node (sx-render-with-env source extra-env)))
-          (hoist-head-elements-full el)
+            (dom-set-text-content el "")
-          ;; Process sx- attributes, hydrate data-sx and islands
+            (dom-append el node)
-          (process-elements el)
+            ;; Hoist head elements from rendered content
-          (sx-hydrate-elements el)
+            (hoist-head-elements-full el)))
-          (sx-hydrate-islands el)
+        ;; Process sx- attributes, hydrate data-sx and islands
-          (run-post-render-hooks))))))
+        (process-elements el)
+        (sx-hydrate-elements el)
+        (sx-hydrate-islands el)
+        (run-post-render-hooks)))))
 
 
 ;; --------------------------------------------------------------------------