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>

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