fix(desktop): let the glass background show through the community rail

The glass treatment (#5478) clears the background on every navigation
surface — app sidebar, settings sidebar, pinned header, footer — so the
native vibrancy shows through, but the community rail's opaque
`bg-sidebar` paint was left out of the `:root[data-glass-background]`
transparency list. With multiple communities the rail rendered as a
solid column while the sidebar next to it frosted, splitting the nav
into two visibly different panes. The Buzz-theme gradient rule already
includes the rail selector; the glass rule simply forgot it.

Add the rail to the glass transparency list and a Playwright regression
spec that fails on the unpatched build: with glass enabled and two
communities seeded, the rail's computed background must be transparent.

Co-authored-by: Thomas Petersen <thomasp@squareup.com>
Signed-off-by: Thomas Petersen <thomasp@squareup.com>

desktop/playwright.config.ts

@@ -19,6 +19,7 @@ export default defineConfig({
     {
       name: "smoke",
       testMatch: [
+        "**/glass-rail.spec.ts",
         "**/smoke.spec.ts",
         "**/sidebar-offcanvas-rail.spec.ts",
         "**/search-scope-screenshots.spec.ts",

desktop/src/shared/styles/globals/theme.css

@@ -832,6 +832,7 @@
 :root[data-glass-background] .group\/sidebar-wrapper,
 :root[data-glass-background] [data-testid="app-sidebar"],
 :root[data-glass-background] [data-testid="settings-sidebar"],
+:root[data-glass-background] [data-testid="community-rail"].bg-sidebar,
 :root[data-glass-background] [data-buzz-glass-inset],
 :root[data-glass-background] [data-buzz-glass-footer-wrap],
 :root[data-glass-background]

desktop/tests/e2e/glass-rail.spec.ts

@@ -0,0 +1,52 @@
+import { expect, test, type Page } from "@playwright/test";
+
+import { installMockBridge } from "../helpers/bridge";
+
+/**
+ * Glass background must clear the community rail's opaque `bg-sidebar` paint
+ * so the native vibrancy shows through the whole navigation column, not just
+ * the channel sidebar. Regression test for the rail being left out of the
+ * `:root[data-glass-background]` transparency list (theme.css).
+ */
+async function seedGlassOnMac(page: Page) {
+  await page.addInitScript(() => {
+    window.localStorage.setItem("buzz-theme", "github-light");
+    window.localStorage.setItem("buzz-glass-background", "true");
+    (window as typeof window & { isTauri?: boolean }).isTauri = true;
+    Object.defineProperty(navigator, "platform", {
+      configurable: true,
+      get: () => "MacIntel",
+    });
+  });
+}
+
+/** Registered AFTER the bridge so it can append to the seeded community list. */
+async function appendSecondCommunity(page: Page) {
+  await page.addInitScript(() => {
+    const raw = window.localStorage.getItem("buzz-communities");
+    const list = raw ? (JSON.parse(raw) as Array<unknown>) : [];
+    list.push({
+      id: "ws-b",
+      name: "Bravo",
+      relayUrl: "ws://localhost:3001",
+      addedAt: "2026-01-02T00:00:00.000Z",
+    });
+    window.localStorage.setItem("buzz-communities", JSON.stringify(list));
+  });
+}
+
+test("glass background clears the community rail surface", async ({ page }) => {
+  await seedGlassOnMac(page);
+  await installMockBridge(page);
+  await appendSecondCommunity(page);
+  await page.goto("/", { waitUntil: "domcontentloaded" });
+
+  await expect(page.locator("html")).toHaveAttribute(
+    "data-glass-background",
+    "",
+  );
+
+  const rail = page.getByTestId("community-rail");
+  await expect(rail).toBeVisible();
+  await expect(rail).toHaveCSS("background-color", "rgba(0, 0, 0, 0)");
+});