Fix pathless relative routing with a basename (#10433)

.changeset/empty-path-basename-navigation.md

@@ -0,0 +1,5 @@
+---
+"@remix-run/router": patch
+---
+
+Fix basename handling when navigating without a path

packages/router/__tests__/router-test.ts

@@ -16220,5 +16220,29 @@ describe("a router", () => {
       expect(createPath(router.state.location)).toBe("/path");
       expect(router.state.matches[2].route.index).toBe(true);
     });
+
+    it("handles pathless relative routing when a basename is present", () => {
+      let router = createRouter({
+        routes: [{ path: "/path" }],
+        future: { v7_prependBasename: true },
+        history: createMemoryHistory({ initialEntries: ["/base/path"] }),
+        basename: "/base",
+      }).initialize();
+
+      expect(createPath(router.state.location)).toBe("/base/path");
+      expect(router.state.matches[0].route.path).toBe("/path");
+
+      router.navigate(".?a=1");
+      expect(createPath(router.state.location)).toBe("/base/path?a=1");
+      expect(router.state.matches[0].route.path).toBe("/path");
+
+      router.navigate("?b=2");
+      expect(createPath(router.state.location)).toBe("/base/path?b=2");
+      expect(router.state.matches[0].route.path).toBe("/path");
+
+      router.navigate("/path?c=3");
+      expect(createPath(router.state.location)).toBe("/base/path?c=3");
+      expect(router.state.matches[0].route.path).toBe("/path");
+    });
   });
 });

packages/router/router.ts

@@ -3094,7 +3094,7 @@ function normalizeTo(
   let path = resolveTo(
     to ? to : ".",
     getPathContributingMatches(contextualMatches).map((m) => m.pathnameBase),
-    location.pathname,
+    stripBasename(location.pathname, basename) || location.pathname,
     relative === "path"
   );