diff --git a/frontend/editor/src/core/tests/helpers/stub-test-base.ts b/frontend/editor/src/core/tests/helpers/stub-test-base.ts
index 5752108d42..e6ae98c6f7 100644
--- a/frontend/editor/src/core/tests/helpers/stub-test-base.ts
+++ b/frontend/editor/src/core/tests/helpers/stub-test-base.ts
@@ -6,6 +6,7 @@ import {
   skipOnboarding,
   type MockAppApiOptions,
 } from "@app/tests/helpers/api-stubs";
+import { suppressNativeFilePicker } from "@app/tests/helpers/ui-helpers";
 
 /**
  * Custom Playwright fixture for backend-free specs.
@@ -57,6 +58,7 @@ export const test = base.extend<StubFixtures>({
   seedJwt: [false, { option: true }],
 
   page: async ({ page, stubOptions, autoGoto, seedJwt }, use) => {
+    suppressNativeFilePicker(page);
     await seedCookieConsent(page);
     if (seedJwt) {
       // Logged-in users hit the orchestrator path that surfaces the
diff --git a/frontend/editor/src/core/tests/helpers/test-base.ts b/frontend/editor/src/core/tests/helpers/test-base.ts
index 1659bc3abd..90de9432f9 100644
--- a/frontend/editor/src/core/tests/helpers/test-base.ts
+++ b/frontend/editor/src/core/tests/helpers/test-base.ts
@@ -1,6 +1,7 @@
 import { test as base, expect } from "@playwright/test";
 import * as fs from "node:fs/promises";
 import * as path from "node:path";
+import { suppressNativeFilePicker } from "@app/tests/helpers/ui-helpers";
 
 /**
  * Custom test fixture that:
@@ -40,6 +41,7 @@ const COVERAGE_DIR = path.resolve(
 
 export const test = base.extend({
   page: async ({ page }, use, testInfo) => {
+    suppressNativeFilePicker(page);
     await page.context().addCookies([
       {
         name: "cc_cookie",
diff --git a/frontend/editor/src/core/tests/helpers/ui-helpers.ts b/frontend/editor/src/core/tests/helpers/ui-helpers.ts
index 7c08f93f60..353037c106 100644
--- a/frontend/editor/src/core/tests/helpers/ui-helpers.ts
+++ b/frontend/editor/src/core/tests/helpers/ui-helpers.ts
@@ -11,6 +11,37 @@ import { expect, type Page, type Locator } from "@playwright/test";
 
 const MANTINE_MODAL_OVERLAY = ".mantine-Modal-overlay";
 
+/**
+ * Suppress the native OS file picker for the whole page, on every browser.
+ *
+ * Several upload entry points (the FileSidebar "Open from computer" button,
+ * the Mantine `<FileInput>`, AddFileCard, etc.) open a file dialog by clicking
+ * a hidden `<input type="file">`. On firefox/webkit Playwright only intercepts
+ * that dialog while the page has a `filechooser` listener - it toggles
+ * `Page.setInterceptFileChooserDialog` off the event subscription. With no
+ * listener the real OS picker leaks onto the host and hangs the nightly run.
+ *
+ * Registering a (no-op) `filechooser` listener flips that interception on for
+ * every browser, so the dialog is suppressed at the browser level however it
+ * was triggered - a programmatic `.click()`, a `<label>` activation, or
+ * Playwright's own click. We deliberately don't set files in the handler: specs
+ * still drive uploads explicitly via `setInputFiles()`, which sets files
+ * through the protocol regardless of the pending intercepted chooser. This lets
+ * a spec click the real entry-point button while the picker stays mocked
+ * cross-browser - unlike a global `HTMLInputElement.prototype.click` override,
+ * which misses `<label>`-triggered pickers and never enables Playwright's own
+ * interception.
+ *
+ * Installed once per page by the shared test fixtures (stub + live), so no spec
+ * has to opt in.
+ */
+export function suppressNativeFilePicker(page: Page): void {
+  page.on("filechooser", () => {
+    // Interception alone suppresses the native dialog; specs provide the files
+    // themselves via setInputFiles() on the hidden input.
+  });
+}
+
 /**
  * Wait for a Mantine Modal overlay to appear or disappear. Most file pickers,
  * settings dialogs, encrypted-PDF unlock prompts and so on render through
@@ -39,8 +70,10 @@ export async function waitForModalClose(
 /**
  * Upload one or more files through the FileSidebar's "Open from computer"
  * action. The button is always rendered (collapsed or expanded sidebar) and
- * triggers the hidden `data-testid="file-input"` native picker directly -
- * there is no modal to wait for under the post-refactor design.
+ * fires the hidden `data-testid="file-input"`. Its native OS picker is mocked
+ * globally by `suppressNativeFilePicker` (installed by the test fixtures), so
+ * the click is safe on every browser; we then set the files directly on the
+ * input via `setInputFiles`.
  *
  * `setInputFiles` doesn't await the input's async onChange (which writes to
  * IndexedDB via `addFiles`), so without a sync point a caller that follows
@@ -73,8 +106,15 @@ export async function switchToEditorIfViewerMode(page: Page): Promise<void> {
   const goToEditor = page.getByRole("button", {
     name: /go to file editor/i,
   });
+  // The affordance only exists while the workbench is transiently in viewer
+  // mode after an upload. The app can auto-leave viewer mode and detach the
+  // button between our visibility check and the click - the transition timing
+  // differs on firefox/webkit, where the detached button hangs a plain
+  // `click()` for the full actionability timeout. Treat a vanished button as
+  // "already in editor mode": swallow the click failure and let the caller's
+  // run-button assertion catch any genuine regression.
   if (await goToEditor.isVisible({ timeout: 1_000 }).catch(() => false)) {
-    await goToEditor.click();
+    await goToEditor.click({ timeout: 5_000 }).catch(() => {});
   }
 }
 
diff --git a/frontend/editor/src/core/tests/live/encrypted-unlock-then-tool.spec.ts b/frontend/editor/src/core/tests/live/encrypted-unlock-then-tool.spec.ts
index ef03c54369..de9d803d25 100644
--- a/frontend/editor/src/core/tests/live/encrypted-unlock-then-tool.spec.ts
+++ b/frontend/editor/src/core/tests/live/encrypted-unlock-then-tool.spec.ts
@@ -31,6 +31,9 @@ test.describe("Encrypted PDF: unlock then merge", () => {
     await page.goto("/merge");
     await page.waitForLoadState("domcontentloaded");
 
+    // `files-button`'s native picker is mocked globally
+    // (suppressNativeFilePicker), so the click is safe cross-browser; set the
+    // files on the hidden input directly.
     await page.getByTestId("files-button").click();
     await page
       .locator('[data-testid="file-input"]')
diff --git a/frontend/editor/src/core/tests/stubbed/certificate-validation.spec.ts b/frontend/editor/src/core/tests/stubbed/certificate-validation.spec.ts
index 1f8886bab4..075464c587 100644
--- a/frontend/editor/src/core/tests/stubbed/certificate-validation.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/certificate-validation.spec.ts
@@ -1,5 +1,6 @@
 import { test, expect, type Page } from "@playwright/test";
 import path from "path";
+import { suppressNativeFilePicker } from "@app/tests/helpers/ui-helpers";
 
 // ---------------------------------------------------------------------------
 // Test fixtures — pre-generated keystores in test-fixtures/certs/
@@ -95,6 +96,10 @@ async function uploadCertFile(page: Page, filePath: string) {
 // ---------------------------------------------------------------------------
 test.describe("Certificate Validation — ParticipantView", () => {
   test.beforeEach(async ({ page }) => {
+    // Raw @playwright/test fixture: install the picker suppression directly so
+    // clicking the Mantine <FileInput> button is intercepted cross-browser
+    // (firefox/webkit otherwise leak the native dialog and close the page).
+    suppressNativeFilePicker(page);
     await mockParticipantApis(page);
   });
 
diff --git a/frontend/editor/src/core/tests/stubbed/convert.spec.ts b/frontend/editor/src/core/tests/stubbed/convert.spec.ts
index cd6a60884a..de8fa17a9c 100644
--- a/frontend/editor/src/core/tests/stubbed/convert.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/convert.spec.ts
@@ -8,6 +8,7 @@
 import { test, expect, type Page } from "@playwright/test";
 import path from "path";
 import { mockAppApis } from "@app/tests/helpers/api-stubs";
+import { suppressNativeFilePicker } from "@app/tests/helpers/ui-helpers";
 
 const FIXTURES_DIR = path.join(__dirname, "../test-fixtures");
 const SAMPLE_PDF = path.join(FIXTURES_DIR, "sample.pdf");
@@ -23,10 +24,10 @@ async function dismissTourTooltip(page: Page) {
 }
 
 // ---------------------------------------------------------------------------
-// Helper: upload a file via the FileSidebar's "Open from computer" action.
-// The button now triggers the native OS picker directly - no modal - and
-// the hidden `data-testid="file-input"` accepts `setInputFiles` in either
-// sidebar state.
+// Helper: upload a file via the FileSidebar's "Open from computer" action. The
+// button's native OS picker is mocked globally by `suppressNativeFilePicker`
+// (test fixtures), so the click is safe cross-browser; the hidden
+// `data-testid="file-input"` then accepts `setInputFiles` in either state.
 // ---------------------------------------------------------------------------
 async function uploadFile(page: Page, filePath: string) {
   await page.getByTestId("files-button").click();
@@ -60,6 +61,11 @@ async function selectToFormat(page: Page, toValue: string) {
 // ---------------------------------------------------------------------------
 test.describe("Convert Tool", () => {
   test.beforeEach(async ({ page }) => {
+    // These specs use the raw @playwright/test fixture, so they don't get the
+    // shared stub-test-base suppression - install it here so the picker click
+    // is intercepted cross-browser (firefox/webkit otherwise leak the native
+    // dialog onto the host and close the page).
+    suppressNativeFilePicker(page);
     await mockAppApis(page);
     await page.goto("/?bypassOnboarding=true");
     await page.waitForSelector('[data-testid="files-button"]', {
diff --git a/frontend/editor/src/core/tests/stubbed/encrypted-pdf-unlock.spec.ts b/frontend/editor/src/core/tests/stubbed/encrypted-pdf-unlock.spec.ts
index 7e220a3bda..2a202b6cfe 100644
--- a/frontend/editor/src/core/tests/stubbed/encrypted-pdf-unlock.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/encrypted-pdf-unlock.spec.ts
@@ -21,6 +21,7 @@ import { test, expect, type Page } from "@playwright/test";
 import path from "path";
 import fs from "fs";
 import { mockAppApis } from "@app/tests/helpers/api-stubs";
+import { suppressNativeFilePicker } from "@app/tests/helpers/ui-helpers";
 
 const FIXTURES_DIR = path.join(__dirname, "../test-fixtures");
 const ENCRYPTED_PDF = path.join(FIXTURES_DIR, "encrypted.pdf");
@@ -63,8 +64,10 @@ function mockRemovePasswordWrongPassword(page: Page) {
 }
 
 async function uploadEncryptedFile(page: Page, filePath: string) {
+  // `files-button`'s native picker is mocked globally
+  // (suppressNativeFilePicker), so the click is safe cross-browser; set the
+  // files on the hidden input directly.
   await page.getByTestId("files-button").click();
-  // No modal flow - `files-button` triggers the native picker directly.
   await page.locator('[data-testid="file-input"]').setInputFiles(filePath);
 }
 
@@ -76,6 +79,10 @@ test.describe.configure({ mode: "serial" });
 
 test.describe("Encrypted PDF Unlock Modal", () => {
   test.beforeEach(async ({ page }) => {
+    // Raw @playwright/test fixture: install the picker suppression directly so
+    // the files-button click is intercepted cross-browser (firefox/webkit
+    // otherwise leak the native dialog onto the host and close the page).
+    suppressNativeFilePicker(page);
     await mockAppApis(page);
     await page.goto("/?bypassOnboarding=true");
     await page.waitForSelector('[data-testid="files-button"]', {
@@ -153,8 +160,9 @@ test.describe("Encrypted PDF Unlock Modal", () => {
   }) => {
     await mockRemovePasswordSuccess(page);
 
+    // `files-button`'s native picker is mocked globally; click it, then set
+    // the files on the hidden input directly.
     await page.getByTestId("files-button").click();
-    // No modal flow - `files-button` triggers the native picker directly.
     await page.locator('[data-testid="file-input"]').setInputFiles([
       {
         name: "encrypted-a.pdf",
diff --git a/frontend/editor/src/core/tests/stubbed/files-page-screenshots.spec.ts b/frontend/editor/src/core/tests/stubbed/files-page-screenshots.spec.ts
index fffa1b4b65..f4948945fe 100644
--- a/frontend/editor/src/core/tests/stubbed/files-page-screenshots.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/files-page-screenshots.spec.ts
@@ -445,8 +445,17 @@ test.describe("Files page screenshots", () => {
       localStorage.setItem("i18nextLng", "ar-AR");
       localStorage.setItem("stirling-language", "ar-AR");
       localStorage.setItem("stirling-language-source", "user");
-      document.documentElement.setAttribute("dir", "rtl");
-      document.documentElement.setAttribute("lang", "ar-AR");
+      // On webkit, `document.documentElement` is still null when Playwright
+      // runs init scripts, so calling setAttribute directly throws - and that
+      // uncaught error aborts the *following* init script (the IndexedDB seed
+      // in seedFiles), leaving the grid stuck on skeletons. Guard the access
+      // and defer to DOMContentLoaded when the element isn't there yet.
+      const applyDir = () => {
+        document.documentElement.setAttribute("dir", "rtl");
+        document.documentElement.setAttribute("lang", "ar-AR");
+      };
+      if (document.documentElement) applyDir();
+      else document.addEventListener("DOMContentLoaded", applyDir);
     });
   }
 
diff --git a/frontend/editor/src/core/tests/stubbed/pdf-text-search.spec.ts b/frontend/editor/src/core/tests/stubbed/pdf-text-search.spec.ts
index 5ba1599386..ce4bbce081 100644
--- a/frontend/editor/src/core/tests/stubbed/pdf-text-search.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/pdf-text-search.spec.ts
@@ -16,8 +16,9 @@ test.describe("Reader - in-document text search", () => {
     await page.goto("/read");
     await page.waitForLoadState("domcontentloaded");
 
-    // Upload a PDF first so the reader has content. `files-button` now
-    // triggers the native picker directly - no modal flow involved.
+    // Upload a PDF first so the reader has content. The `files-button` native
+    // picker is mocked globally (suppressNativeFilePicker), so the click is
+    // safe cross-browser; set the files on the hidden input directly.
     await page.getByTestId("files-button").click();
     await page.locator('[data-testid="file-input"]').setInputFiles(SAMPLE_PDF);
 
diff --git a/frontend/editor/src/core/tests/stubbed/viewer-text-selection.spec.ts b/frontend/editor/src/core/tests/stubbed/viewer-text-selection.spec.ts
index 573789aca3..1a7486e37a 100644
--- a/frontend/editor/src/core/tests/stubbed/viewer-text-selection.spec.ts
+++ b/frontend/editor/src/core/tests/stubbed/viewer-text-selection.spec.ts
@@ -96,7 +96,15 @@ test("hovering over text changes the cursor to an I-beam", async ({ page }) => {
 test("Ctrl+C copies selected text to the clipboard", async ({
   page,
   context,
+  browserName,
 }) => {
+  // Reading the clipboard requires the `clipboard-read` permission, which only
+  // chromium supports via `grantPermissions` (firefox throws "Unknown
+  // permission"; webkit can't expose `navigator.clipboard.readText` in tests).
+  test.skip(
+    browserName !== "chromium",
+    "clipboard read/permissions are chromium-only in Playwright",
+  );
   test.setTimeout(60_000);
   await context.grantPermissions(["clipboard-read", "clipboard-write"]);
   const firstPage = await loadSampleAndOpenViewer(page);
@@ -169,7 +177,15 @@ test("right-click on the page does not surface the browser context menu", async
 test("floating Copy menu appears after drag-select and copies", async ({
   page,
   context,
+  browserName,
 }) => {
+  // Verifying the copy result reads the clipboard, which needs the
+  // `clipboard-read` permission - chromium-only in Playwright. The Copy menu's
+  // appearance is covered cross-browser by the right-click test above.
+  test.skip(
+    browserName !== "chromium",
+    "clipboard read/permissions are chromium-only in Playwright",
+  );
   test.setTimeout(60_000);
   await context.grantPermissions(["clipboard-read", "clipboard-write"]);
   const firstPage = await loadSampleAndOpenViewer(page);