diff --git a/package-lock.json b/package-lock.json
index 2df5720..8dbeca5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,7 +9,7 @@
       "version": "0.0.1",
       "dependencies": {
         "@intechstudio/grid-protocol": "1.20260828.1315",
-        "@intechstudio/grid-uikit": "1.20260922.1223",
+        "@intechstudio/grid-uikit": "1.20261014.1208",
         "@melt-ui/svelte": "^0.86.6",
         "firebase": "^12.2.1",
         "marked": "^12.0.2",
@@ -1255,18 +1255,28 @@
       }
     },
     "node_modules/@intechstudio/grid-uikit": {
-      "version": "1.20260922.1223",
-      "resolved": "https://registry.npmjs.org/@intechstudio/grid-uikit/-/grid-uikit-1.20260922.1223.tgz",
-      "integrity": "sha512-GEJICGrecYwLgCBLA+6Luz03O1Jrej13OoHN7mEohpkgsQV2+FW8TTYuSL4Z1eDfbY8Bs1JCrX3IgzXFrg6fCA==",
+      "version": "1.20261014.1208",
+      "resolved": "https://registry.npmjs.org/@intechstudio/grid-uikit/-/grid-uikit-1.20261014.1208.tgz",
+      "integrity": "sha512-Lvqp2VujI/GCOh92rQfGXxoYcLa1bDqRxPsthlDj/GrjfZUDdosI4h5EIDGSVpvYmeLUiaxead6UL2n9GXt+Og==",
       "dependencies": {
         "@melt-ui/svelte": "^0.86.6",
         "color-convert": "^2.0.1",
-        "svelte-easy-popover": "^1.0.0"
+        "svelte-easy-popover": "^1.0.0",
+        "svelte-splitpanes": "^8.0.12"
       },
       "peerDependencies": {
         "svelte": "^4.2.12 || ^5.1.0"
       }
     },
+    "node_modules/@intechstudio/grid-uikit/node_modules/svelte-splitpanes": {
+      "version": "8.0.12",
+      "resolved": "https://registry.npmjs.org/svelte-splitpanes/-/svelte-splitpanes-8.0.12.tgz",
+      "integrity": "sha512-HJ07HgbtY0Q/35TEuJquGy47dtgCVavV7ay9r1FhWRx3boyUs3RpeiHlwMKliznCKy2ZotNeT+8GG+mIoNjgRA==",
+      "license": "MIT",
+      "peerDependencies": {
+        "svelte": "^5.43.0"
+      }
+    },
     "node_modules/@internationalized/date": {
       "version": "3.9.0",
       "resolved": "https://registry.npmjs.org/@internationalized/date/-/date-3.9.0.tgz",
@@ -1306,9 +1316,9 @@
       }
     },
     "node_modules/@jridgewell/sourcemap-codec": {
-      "version": "1.5.5",
-      "resolved": "https://registry.npmjs.org/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.5.5.tgz",
-      "integrity": "sha512-cYQ9310grqxueWbl+WuIUIaiUaDcj7WOq5fVhEljNVgRfOUhY9fy2zTvfoqWsnebh8Sl70VScFbICvJnLKB0Og==",
+      "version": "1.6.0",
+      "resolved": "https://registry.npmjs.org/@jridgewell/sourcemap-codec/-/sourcemap-codec-1.6.0.tgz",
+      "integrity": "sha512-T7jf+5zgsZHwNJ4lvQ7/aezbyk0nNX+zJVWpmHA7VYsEx7a7qr5Rg5IbtJFqkgze5Y2sruq1RUY8Q837Od7iFw==",
       "license": "MIT"
     },
     "node_modules/@jridgewell/trace-mapping": {
@@ -1773,9 +1783,9 @@
       "license": "MIT"
     },
     "node_modules/@sveltejs/acorn-typescript": {
-      "version": "1.0.6",
-      "resolved": "https://registry.npmjs.org/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.6.tgz",
-      "integrity": "sha512-4awhxtMh4cx9blePWl10HRHj8Iivtqj+2QdDCSMDzxG+XKa9+VCNupQuCuvzEhYPzZSrX+0gC+0lHA/0fFKKQQ==",
+      "version": "1.0.13",
+      "resolved": "https://registry.npmjs.org/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.13.tgz",
+      "integrity": "sha512-wgKggnhZVL9Bfx1OaKKTrYY9BFRk6C8UAkQNUcIv1+llzYrIqy+RZm5HPKzn0NpEBvTVhTqB4kQyllZywsRBRQ==",
       "license": "MIT",
       "peerDependencies": {
         "acorn": "^8.9.0"
@@ -2147,9 +2157,9 @@
       "license": "MIT"
     },
     "node_modules/acorn": {
-      "version": "8.15.0",
-      "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.15.0.tgz",
-      "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
+      "version": "8.19.0",
+      "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.19.0.tgz",
+      "integrity": "sha512-oJlA3XiRm7Cyk6qFD2Jn8ak9B7jSy0qy00ADO3+8dpT0LSjFihQYv4C02LFCSYJV3Q37xYwwRy5m+IpIiUzqWw==",
       "license": "MIT",
       "bin": {
         "acorn": "bin/acorn"
@@ -2217,9 +2227,9 @@
       "license": "Python-2.0"
     },
     "node_modules/aria-query": {
-      "version": "5.3.2",
-      "resolved": "https://registry.npmjs.org/aria-query/-/aria-query-5.3.2.tgz",
-      "integrity": "sha512-COROpnaoap1E2F000S62r6A60uHZnmlvomhfyT2DlTcrY1OrBKn2UhH7qn5wTC9zMvD0AY7csdPSNwKP+7WiQw==",
+      "version": "5.3.1",
+      "resolved": "https://registry.npmjs.org/aria-query/-/aria-query-5.3.1.tgz",
+      "integrity": "sha512-Z/ZeOgVl7bcSYZ/u/rh0fOpvEpq//LZmdbkXyc7syVzjPAhfOa9ebsdTSjEBDU4vs5nC98Kfduj1uFo0qyET3g==",
       "license": "Apache-2.0",
       "engines": {
         "node": ">= 0.4"
@@ -2434,10 +2444,9 @@
       }
     },
     "node_modules/devalue": {
-      "version": "5.6.2",
-      "resolved": "https://registry.npmjs.org/devalue/-/devalue-5.6.2.tgz",
-      "integrity": "sha512-nPRkjWzzDQlsejL1WVifk5rvcFi/y1onBRxjaFMjZeR9mFpqu2gmAZ9xUB9/IEanEP/vBtGeGganC/GO1fmufg==",
-      "dev": true,
+      "version": "5.9.4",
+      "resolved": "https://registry.npmjs.org/devalue/-/devalue-5.9.4.tgz",
+      "integrity": "sha512-sPAT4pztbu6586/hrhOnMKS17IJrvg12mXiSPSS3W5qDeN2RGgvZ0diZCm31dBbnevfVmujNO3IM2wrS4Y2Rhg==",
       "license": "MIT"
     },
     "node_modules/dir-glob": {
@@ -2781,15 +2790,6 @@
         "node": ">=4.0"
       }
     },
-    "node_modules/esrap": {
-      "version": "2.1.0",
-      "resolved": "https://registry.npmjs.org/esrap/-/esrap-2.1.0.tgz",
-      "integrity": "sha512-yzmPNpl7TBbMRC5Lj2JlJZNPml0tzqoqP5B1JXycNUwtqma9AKCO0M2wHrdgsHcy1WRW7S9rJknAMtByg3usgA==",
-      "license": "MIT",
-      "dependencies": {
-        "@jridgewell/sourcemap-codec": "^1.4.15"
-      }
-    },
     "node_modules/esrecurse": {
       "version": "4.3.0",
       "resolved": "https://registry.npmjs.org/esrecurse/-/esrecurse-4.3.0.tgz",
@@ -4155,21 +4155,22 @@
       }
     },
     "node_modules/svelte": {
-      "version": "5.39.8",
-      "resolved": "https://registry.npmjs.org/svelte/-/svelte-5.39.8.tgz",
-      "integrity": "sha512-KfZ3hCITdxIXTOvrea4nFZX2o+47HPTChKeocgj9BwJQYqWrviVCcPj4boXHF5yf8+eBKqhHY8xii//XaakKXA==",
+      "version": "5.57.2",
+      "resolved": "https://registry.npmjs.org/svelte/-/svelte-5.57.2.tgz",
+      "integrity": "sha512-1YiPCAXT0Ct6g1vOMmSLa0Nsc3QRMvtDF32ihgglGVrugIuIK38RyDiFNXVDhtQ81TJjSO57NdMw6DyYLqSGDQ==",
       "license": "MIT",
       "dependencies": {
-        "@jridgewell/remapping": "^2.3.4",
-        "@jridgewell/sourcemap-codec": "^1.5.0",
-        "@sveltejs/acorn-typescript": "^1.0.5",
-        "@types/estree": "^1.0.5",
-        "acorn": "^8.12.1",
-        "aria-query": "^5.3.1",
+        "@jridgewell/remapping": "^2.3.5",
+        "@jridgewell/sourcemap-codec": "^1.6.0",
+        "@sveltejs/acorn-typescript": "^1.0.13",
+        "@types/estree": "^1.0.9",
+        "acorn": "^8.18.0",
+        "aria-query": "5.3.1",
         "axobject-query": "^4.1.0",
         "clsx": "^2.1.1",
+        "devalue": "^5.9.2",
         "esm-env": "^1.2.1",
-        "esrap": "^2.1.0",
+        "esrap": "^2.3.6",
         "is-reference": "^3.0.3",
         "locate-character": "^3.0.0",
         "magic-string": "^0.30.11",
@@ -4382,6 +4383,29 @@
         "svelte": "^4.2.19 || ^5.1.0"
       }
     },
+    "node_modules/svelte/node_modules/@types/estree": {
+      "version": "1.0.9",
+      "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.9.tgz",
+      "integrity": "sha512-GhdPgy1el4/ImP05X05Uw4cw2/M93BCUmnEvWZNStlCzEKME4Fkk+YpoA5OiHNQmoS7Cafb8Xa3Pya8m1Qrzeg==",
+      "license": "MIT"
+    },
+    "node_modules/svelte/node_modules/esrap": {
+      "version": "2.4.0",
+      "resolved": "https://registry.npmjs.org/esrap/-/esrap-2.4.0.tgz",
+      "integrity": "sha512-CQPcneEHw/czj32uwQC52nYnax8H4SXt25FCM+LIwGZ49cMIxpOj3bP8wmzcI+Z0HIy5KkClaPhRLfCySzwHPA==",
+      "license": "MIT",
+      "dependencies": {
+        "@jridgewell/sourcemap-codec": "^1.4.15"
+      },
+      "peerDependencies": {
+        "@typescript-eslint/types": "^8.2.0"
+      },
+      "peerDependenciesMeta": {
+        "@typescript-eslint/types": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/tabbable": {
       "version": "6.2.0",
       "resolved": "https://registry.npmjs.org/tabbable/-/tabbable-6.2.0.tgz",
diff --git a/package.json b/package.json
index ae8a0dc..cb083f8 100644
--- a/package.json
+++ b/package.json
@@ -37,7 +37,7 @@
   "type": "module",
   "dependencies": {
     "@intechstudio/grid-protocol": "1.20260828.1315",
-    "@intechstudio/grid-uikit": "1.20260922.1223",
+    "@intechstudio/grid-uikit": "1.20261014.1208",
     "@melt-ui/svelte": "^0.86.6",
     "firebase": "^12.2.1",
     "marked": "^12.0.2",
diff --git a/src/app-environment-shim.ts b/src/app-environment-shim.ts
new file mode 100644
index 0000000..e8fac33
--- /dev/null
+++ b/src/app-environment-shim.ts
@@ -0,0 +1,7 @@
+// Shim for $app/environment (a SvelteKit-only module) so svelte-splitpanes
+// resolves outside SvelteKit. The webcomponent build is plain Vite, not a
+// SvelteKit app, so this module doesn't exist there otherwise.
+export const browser = true;
+export const building = false;
+export const dev = import.meta.env.DEV;
+export const version = "";
diff --git a/vite.config.webcomponent.ts b/vite.config.webcomponent.ts
index 5f64a3a..363b1c7 100644
--- a/vite.config.webcomponent.ts
+++ b/vite.config.webcomponent.ts
@@ -2,8 +2,35 @@ import { defineConfig } from "vite";
 import { svelte } from "@sveltejs/vite-plugin-svelte";
 import { sveltePreprocess } from "svelte-preprocess";
 import { notifyEditorAfterBuildPlugin } from "./vite.postbuild";
+import path from "path";
 
 export default defineConfig({
+  resolve: {
+    alias: {
+      // svelte-splitpanes (pulled in transitively via grid-uikit's
+      // GridSplitpanes) imports the SvelteKit-only $app/environment module;
+      // this build isn't a SvelteKit app, so shim it. Same pattern as
+      // grid-uikit's own demo and grid-editor's renderer config.
+      "$app/environment": path.resolve(
+        __dirname,
+        "src/app-environment-shim.ts",
+      ),
+    },
+  },
+  optimizeDeps: {
+    esbuildOptions: {
+      plugins: [
+        {
+          name: "resolve-app-environment",
+          setup(build) {
+            build.onResolve({ filter: /^\$app\/environment$/ }, () => ({
+              path: path.resolve(__dirname, "src/app-environment-shim.ts"),
+            }));
+          },
+        },
+      ],
+    },
+  },
   plugins: [
     svelte({
       preprocess: sveltePreprocess({