diff --git a/apps/desktop/src/window/DesktopWindow.test.ts b/apps/desktop/src/window/DesktopWindow.test.ts
index 8b6ab7c7d8a5e..16d345f97e4b6 100644
--- a/apps/desktop/src/window/DesktopWindow.test.ts
+++ b/apps/desktop/src/window/DesktopWindow.test.ts
@@ -472,6 +472,45 @@ describe("DesktopWindow", () => {
     );
   });
 
+  it("rejects renderer navigations whose origin only serializes as null", () => {
+    for (const applicationUrl of ["t3code://app/", "t3code-dev://app/"]) {
+      assert.isTrue(
+        DesktopWindow.isSameOriginRendererNavigation({
+          applicationUrl,
+          navigationUrl: `${applicationUrl}threads?id=1#latest`,
+        }),
+        applicationUrl,
+      );
+    }
+    for (const navigationUrl of [
+      "t3code://other/",
+      "t3code://app.example/",
+      "t3code://app:8080/",
+      "t3code://user@other/",
+      "t3code:app",
+      "t3code-dev://app/",
+      "file:///etc/passwd",
+      "data:text/html,<p>hi</p>",
+      "javascript:alert(1)",
+      "about:blank",
+      "blob:t3code://app/0b5e2c4a",
+    ]) {
+      assert.isFalse(
+        DesktopWindow.isSameOriginRendererNavigation({
+          applicationUrl: "t3code://app/",
+          navigationUrl,
+        }),
+        navigationUrl,
+      );
+    }
+    assert.isFalse(
+      DesktopWindow.isSameOriginRendererNavigation({
+        applicationUrl: "data:text/html,app",
+        navigationUrl: "data:text/html,other",
+      }),
+    );
+  });
+
   it.effect(
     "opens and reopens the window without backend readiness when local execution is disabled",
     () =>
@@ -973,6 +1012,17 @@ describe("DesktopWindow", () => {
         if (!willNavigate) {
           return yield* Effect.die("will-navigate listener was not registered");
         }
+        for (const [url, expectedPrevented] of [
+          ["t3code-dev://app/threads?id=1#latest", false],
+          ["t3code-dev://other/", true],
+          ["t3code://app/", true],
+        ] as const) {
+          const preventDefault = vi.fn();
+          willNavigate({ preventDefault }, url);
+          assert.equal(preventDefault.mock.calls.length, expectedPrevented ? 1 : 0, url);
+        }
+        assert.deepEqual(openedExternalUrls, []);
+
         let prevented = false;
         willNavigate(
           {
diff --git a/apps/desktop/src/window/DesktopWindow.ts b/apps/desktop/src/window/DesktopWindow.ts
index 6270f80a4a7e6..2a6353cfa9479 100644
--- a/apps/desktop/src/window/DesktopWindow.ts
+++ b/apps/desktop/src/window/DesktopWindow.ts
@@ -182,12 +182,19 @@ function buildConnectingSplashDataUrl(shouldUseDarkColors: boolean): string {
   return `data:text/html;charset=utf-8,${encodeURIComponent(html)}`;
 }
 
-function isSameOriginRendererNavigation(input: {
+export function isSameOriginRendererNavigation(input: {
   readonly applicationUrl: string;
   readonly navigationUrl: string;
 }): boolean {
   try {
-    return new URL(input.applicationUrl).origin === new URL(input.navigationUrl).origin;
+    const applicationUrl = new URL(input.applicationUrl);
+    const navigationUrl = new URL(input.navigationUrl);
+    // Node serializes custom-scheme origins as "null", even for different hosts and schemes.
+    return (
+      applicationUrl.host !== "" &&
+      navigationUrl.protocol === applicationUrl.protocol &&
+      navigationUrl.host === applicationUrl.host
+    );
   } catch {
     return false;
   }