avoid injecting duplicate instances into the 404 page

src/index.ts

@@ -1,46 +1,59 @@
 import App from "./App.svelte";
 import { testPaths } from "./check-url";
-import { CONTAINER_PARENT_SELECTOR } from "./constants";
-
-const parent = document.querySelector(CONTAINER_PARENT_SELECTOR);
-
-const segments = [
-  {
-    content: "github.com",
-    left: 0,
-    right: 0,
-    hovered: false,
-  },
-  ...window.location.pathname
-    .split("/")
-    .filter((segment) => segment !== "")
-    .map((segment) => ({
-      content: segment,
+import { CLASS_PREFIX, CONTAINER_PARENT_SELECTOR } from "./constants";
+
+const mainClassname = CLASS_PREFIX + "main";
+
+const main = () => {
+  if (document.querySelector("." + mainClassname)) {
+    // Already injected content, nothing to do. This can occur when navigating
+    // with the browser back button to a 404 page.
+    return;
+  }
+
+  const div = document.createElement("div");
+  div.classList.add(mainClassname);
+
+  const parent = document.querySelector(CONTAINER_PARENT_SELECTOR);
+  parent.insertAdjacentElement("afterend", div);
+
+  const segments = [
+    {
+      content: "github.com",
       left: 0,
       right: 0,
       hovered: false,
-    })),
-];
-
-const div = document.createElement("div");
-parent.insertAdjacentElement("afterend", div);
-
-const app = new App({
-  target: div,
-  props: { segments, status: "loading", index: 1 },
-});
-
-const onPathTested = (index: number, ok: boolean) => {
-  if (ok) {
-    // Given index was ok, move to the next segment. Shifted by one extra due to
-    // `github.com` prefix.
-    app.$set({ index: index + 2, status: "loading" });
-  } else {
-    // Given index returned 404, mark it as errored. Shifted by one extra due to
-    // `github.com` prefix.
-    app.$set({ index: index + 1, status: "error" });
-  }
+    },
+    ...window.location.pathname
+      .split("/")
+      .filter((segment) => segment !== "")
+      .map((segment) => ({
+        content: segment,
+        left: 0,
+        right: 0,
+        hovered: false,
+      })),
+  ];
+
+  const app = new App({
+    target: div,
+    props: { segments, status: "loading", index: 1 },
+  });
+
+  const onPathTested = (index: number, ok: boolean) => {
+    if (ok) {
+      // Given index was ok, move to the next segment. Shifted by one extra due to
+      // `github.com` prefix.
+      app.$set({ index: index + 2, status: "loading" });
+    } else {
+      // Given index returned 404, mark it as errored. Shifted by one extra due to
+      // `github.com` prefix.
+      app.$set({ index: index + 1, status: "error" });
+    }
+  };
+
+  // TODO catch errors here
+  testPaths(window.location.pathname, onPathTested);
 };
 
-// TODO catch errors here
-testPaths(window.location.pathname, onPathTested);
+main();