diff --git a/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx b/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
index 65be1cfa13..feda337ac1 100644
--- a/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
+++ b/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
@@ -1,5 +1,5 @@
 import { BlockNoteEditor } from "@blocknote/core";
-import { MouseEvent, ReactNode, useId, useRef } from "react";
+import { MouseEvent, ReactNode, useCallback, useId, useRef } from "react";
 import { MdKeyboardReturn } from "react-icons/md";
 
 import { PreviewPlaceholder } from "./PreviewPlaceholder.js";
@@ -95,6 +95,42 @@ export const SourceWithPreview = (
     sourcePlaceholder,
   } = props;
 
+  // Clean up on ref detachment rather than returning a cleanup (React 18 support).
+  const errorCleanupRef = useRef<(() => void) | undefined>(undefined);
+  const errorRef = useCallback(
+    (node: HTMLDivElement | null) => {
+      errorCleanupRef.current?.();
+      errorCleanupRef.current = undefined;
+      if (!node || !popup.isOpen) {
+        return;
+      }
+      const element = node;
+      const document = element.ownerDocument;
+      // These events target the document or the focused editor, not the error
+      // element. Keep ProseMirror from reading a native selection inside the
+      // non-editable error as a document selection or copying the source instead.
+      function stopErrorSelectionEvent(event: Event) {
+        const selection = document.getSelection();
+        if (
+          element.contains(selection?.anchorNode ?? null) &&
+          element.contains(selection?.focusNode ?? null)
+        ) {
+          event.stopImmediatePropagation();
+        }
+      }
+      const events = ["selectionchange", "copy", "cut", "paste", "keydown"];
+      for (const event of events) {
+        document.addEventListener(event, stopErrorSelectionEvent, true);
+      }
+      errorCleanupRef.current = () => {
+        for (const event of events) {
+          document.removeEventListener(event, stopErrorSelectionEvent, true);
+        }
+      };
+    },
+    [popup.isOpen],
+  );
+
   // Whether the error has been "committed": shown while the popup was
   // closed. The last-good preview only stays up within the editing session
   // that introduced the error - once committed, the error state also shows
@@ -234,10 +270,14 @@ export const SourceWithPreview = (
           </div>
         </div>
         <div
+          ref={errorRef}
           id={errorId}
           className="bn-code-block-source-error"
           contentEditable={false}
           style={{ display: error ? "block" : "none" }}
+          // Stop ProseMirror's mouse handler before it can replace the native
+          // text selection. React's bubbling handler runs after ProseMirror's.
+          onMouseDownCapture={(event) => event.stopPropagation()}
           // Announced while editing (the popup is open); silenced & removed
           // from the tree once the popup closes, where the compact error
           // preview takes over.
diff --git a/tests/src/end-to-end/sourcepreview/errorSelection.test.tsx b/tests/src/end-to-end/sourcepreview/errorSelection.test.tsx
new file mode 100644
index 0000000000..f21eb600f1
--- /dev/null
+++ b/tests/src/end-to-end/sourcepreview/errorSelection.test.tsx
@@ -0,0 +1,147 @@
+import { BlockNoteSchema } from "@blocknote/core";
+import "@blocknote/core/fonts/inter.css";
+import { createReactDiagramBlockSpec } from "@blocknote/diagram-block";
+import { BlockNoteView } from "@blocknote/mantine";
+import "@blocknote/mantine/style.css";
+import {
+  createReactInlineMathSpec,
+  createReactMathBlockSpec,
+} from "@blocknote/math-block";
+import { useCreateBlockNote } from "@blocknote/react";
+import { expect, test, vi } from "vite-plus/test";
+import { render } from "vitest-browser-react";
+
+import { browserName, MOD, userEvent } from "../../utils/context.js";
+import { mouseSequence } from "../../utils/mouse.js";
+
+const schema = BlockNoteSchema.create().extend({
+  blockSpecs: {
+    mathBlock: createReactMathBlockSpec(),
+    diagram: createReactDiagramBlockSpec(),
+  },
+  inlineContentSpecs: { math: createReactInlineMathSpec() },
+});
+
+type Kind = "mathBlock" | "diagram" | "math";
+
+function ErrorSelectionApp({ kind }: { kind: Kind }) {
+  const editor = useCreateBlockNote({
+    schema,
+    initialContent:
+      kind === "math"
+        ? [
+            {
+              type: "paragraph",
+              content: [{ type: "math", content: "\\badcommand" }],
+            },
+          ]
+        : [
+            {
+              type: kind,
+              content: kind === "diagram" ? "graph TD\nA -->" : "\\badcommand",
+            },
+          ],
+  });
+  return (
+    <>
+      <BlockNoteView editor={editor} />
+      <textarea aria-label="Paste error text" />
+    </>
+  );
+}
+
+test.each<Kind>(["mathBlock", "diagram", "math"])(
+  "selecting and copying %s error text preserves the popup and source editing",
+  async (kind) => {
+    await render(<ErrorSelectionApp kind={kind} />);
+    await userEvent.click(
+      document.querySelector<HTMLElement>(".bn-preview-container")!,
+    );
+    const error = document.querySelector<HTMLElement>(
+      ".bn-code-block-source-error",
+    )!;
+    await vi.waitFor(() => {
+      expect(error.textContent?.length).toBeGreaterThan(0);
+      expect(
+        error
+          .closest(".bn-preview-with-source-popup")
+          ?.getAttribute("data-open"),
+      ).toBe("true");
+    });
+
+    // Drag across the first word using real mouse events. A DOM-only selection
+    // wouldn't reproduce Firefox resetting the selection after mouseup (#3138).
+    await userEvent.hover(error);
+    const text = error.firstChild!;
+    const word = (text.textContent ?? "").match(/\S+/)![0];
+    const range = document.createRange();
+    range.setStart(text, 0);
+    range.setEnd(text, word.length);
+    const rect = range.getBoundingClientRect();
+    await mouseSequence([
+      { type: "move", x: rect.left, y: rect.top + rect.height / 2 },
+      { type: "down" },
+      { type: "move", x: rect.right, y: rect.top + rect.height / 2, steps: 10 },
+      { type: "up" },
+    ]);
+    await vi.waitFor(() => {
+      expect(window.getSelection()?.toString()).toBe(word);
+      expect(
+        error
+          .closest(".bn-preview-with-source-popup")
+          ?.getAttribute("data-open"),
+      ).toBe("true");
+    });
+    const source = document.querySelector<HTMLElement>(
+      ".bn-source-block-popup code",
+    )!;
+    const originalSource = source.textContent;
+    // The editor still has focus, but its shortcuts must not edit or submit
+    // the source while the native selection belongs to the error text.
+    await userEvent.keyboard("{Enter}{Backspace}");
+    expect(source.textContent).toBe(originalSource);
+    expect(
+      error.closest(".bn-preview-with-source-popup")?.getAttribute("data-open"),
+    ).toBe("true");
+    // Playwright WebKit does not support the native clipboard round trip.
+    // Still exercise selection, focus recovery, and dismissal in that browser.
+    if (browserName !== "webkit") {
+      await userEvent.keyboard(`{${MOD}>}c{/${MOD}}`);
+      await userEvent.keyboard(`{${MOD}>}v{/${MOD}}`);
+      await userEvent.keyboard(`{${MOD}>}x{/${MOD}}`);
+      expect(source.textContent).toBe(originalSource);
+    }
+
+    // Focus leaving the editor must close the popup.
+    const textarea = document.querySelector<HTMLTextAreaElement>("textarea")!;
+    await userEvent.click(textarea);
+    expect(
+      error.closest(".bn-preview-with-source-popup")?.getAttribute("data-open"),
+    ).toBe("false");
+    await userEvent.click(
+      document.querySelector<HTMLElement>(".bn-preview-container")!,
+    );
+    await userEvent.click(error);
+
+    // Moving directly back from the error to the source must keep it editable.
+    await userEvent.click(source);
+    await userEvent.keyboard("{End}x");
+    await vi.waitFor(() =>
+      expect(source.textContent).toBe(originalSource + "x"),
+    );
+    expect(
+      source
+        .closest(".bn-preview-with-source-popup")
+        ?.getAttribute("data-open"),
+    ).toBe("true");
+
+    await userEvent.click(textarea);
+    if (browserName !== "webkit") {
+      await userEvent.keyboard(`{${MOD}>}v{/${MOD}}`);
+      expect(textarea.value).toBe(word);
+    }
+    expect(
+      error.closest(".bn-preview-with-source-popup")?.getAttribute("data-open"),
+    ).toBe("false");
+  },
+);