diff --git a/packages/core/src/editor/Block.css b/packages/core/src/editor/Block.css
index ac367ea619..f528c99f7a 100644
--- a/packages/core/src/editor/Block.css
+++ b/packages/core/src/editor/Block.css
@@ -42,6 +42,12 @@ BASIC STYLES
 
 .bn-inline-content {
   width: 100%;
+  /* Ensure pre-wrap even when a parent node view wrapper (e.g. tiptap's
+     NodeViewWrapper) resets white-space to "normal". Without this, browsers
+     normalize trailing spaces to NBSP on input, which causes ProseMirror to
+     compute a replacement instead of a pure insertion and breaks the
+     suggestion-menu trigger detection (issue #2531). */
+  white-space: pre-wrap;
 }
 
 /*
diff --git a/tests/src/end-to-end/slashmenu/slashmenu-customblock.test.ts b/tests/src/end-to-end/slashmenu/slashmenu-customblock.test.ts
new file mode 100644
index 0000000000..55aef164bc
--- /dev/null
+++ b/tests/src/end-to-end/slashmenu/slashmenu-customblock.test.ts
@@ -0,0 +1,46 @@
+import { expect } from "@playwright/test";
+import { test } from "../../setup/setupScript.js";
+import { ALERT_BLOCK_URL, SLASH_MENU_SELECTOR } from "../../utils/const.js";
+
+// Regression test for https://github.com/TypeCellOS/BlockNote/issues/2531
+// The slash menu should open when "/" is typed after a space inside a custom
+// block (isolating: true, separate contentDOM). Previously the menu failed to
+// open in this scenario.
+test.describe("Slash menu in custom (alert) block – issue #2531", () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto(ALERT_BLOCK_URL);
+    await page.waitForSelector(".bn-editor");
+  });
+
+  test("opens slash menu when / is typed at end of alert block content (no preceding space)", async ({
+    page,
+  }) => {
+    // Click into the editable content area of the alert block
+    const alertContent = page
+      .locator('[data-content-type="alert"]')
+      .first()
+      .locator(".bn-inline-content");
+    await alertContent.click();
+    await page.keyboard.press("End");
+
+    await page.keyboard.type("/");
+    await expect(page.locator(SLASH_MENU_SELECTOR)).toBeVisible();
+  });
+
+  test("opens slash menu when / is typed after a space inside alert block (the regression)", async ({
+    page,
+  }) => {
+    // Click into the editable content area of the alert block
+    const alertContent = page
+      .locator('[data-content-type="alert"]')
+      .first()
+      .locator(".bn-inline-content");
+    await alertContent.click();
+    await page.keyboard.press("End");
+
+    // Type a space first — this is the scenario that broke the menu
+    await page.keyboard.type(" ");
+    await page.keyboard.type("/");
+    await expect(page.locator(SLASH_MENU_SELECTOR)).toBeVisible();
+  });
+});
diff --git a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-chromium-linux.png b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-chromium-linux.png
index 4c78e5e1c0..d471faef1c 100644
Binary files a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-chromium-linux.png and b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-chromium-linux.png differ
diff --git a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-firefox-linux.png b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-firefox-linux.png
index 9cd4c44704..7fba12d8d6 100644
Binary files a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-firefox-linux.png and b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-firefox-linux.png differ
diff --git a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-webkit-linux.png b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-webkit-linux.png
index fdece429b8..68c749782b 100644
Binary files a/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-webkit-linux.png and b/tests/src/end-to-end/static/static.test.ts-snapshots/static-rendering-webkit-linux.png differ
diff --git a/tests/src/utils/const.ts b/tests/src/utils/const.ts
index 04cca1cee6..61cedc194d 100644
--- a/tests/src/utils/const.ts
+++ b/tests/src/utils/const.ts
@@ -39,6 +39,10 @@ export const CUSTOM_BLOCKS_REACT_URL = !process.env.RUN_IN_DOCKER
   ? `http://localhost:${PORT}/custom-schema/react-custom-blocks?hideMenu`
   : `http://host.docker.internal:${PORT}/custom-schema/react-custom-blocks?hideMenu`;
 
+export const ALERT_BLOCK_URL = !process.env.RUN_IN_DOCKER
+  ? `http://localhost:${PORT}/custom-schema/alert-block?hideMenu`
+  : `http://host.docker.internal:${PORT}/custom-schema/alert-block?hideMenu`;
+
 export const PASTE_ZONE_SELECTOR = "#pasteZone";
 
 export const EDITOR_SELECTOR = `.bn-editor`;