diff --git a/packages/core/src/extensions/SourceBlockWithPreview/SourceBlockWithPreview.ts b/packages/core/src/extensions/SourceBlockWithPreview/SourceBlockWithPreview.ts
index c9e8648861..b3de102057 100644
--- a/packages/core/src/extensions/SourceBlockWithPreview/SourceBlockWithPreview.ts
+++ b/packages/core/src/extensions/SourceBlockWithPreview/SourceBlockWithPreview.ts
@@ -183,8 +183,19 @@ export const SourceBlockWithPreviewExtension = createExtension(
           signal,
         });
 
-        const handleBlur = () =>
+        function handleBlur(event: FocusEvent) {
+          // Error text has its own focus target so the browser can select and
+          // copy it. Keep the popup open while interacting with that text.
+          if (
+            event.relatedTarget === dom ||
+            (event.relatedTarget instanceof Element &&
+              dom.contains(event.relatedTarget) &&
+              event.relatedTarget.closest(".bn-code-block-source-error"))
+          ) {
+            return;
+          }
           store.setState((state) => ({ ...state, popupOpen: undefined }));
+        }
         dom.addEventListener("blur", handleBlur, { capture: true, signal });
       },
     };
diff --git a/packages/core/src/extensions/SourceInlineContentWithPreview/SourceInlineContentWithPreview.ts b/packages/core/src/extensions/SourceInlineContentWithPreview/SourceInlineContentWithPreview.ts
index 9ab67e93a7..e298bd38af 100644
--- a/packages/core/src/extensions/SourceInlineContentWithPreview/SourceInlineContentWithPreview.ts
+++ b/packages/core/src/extensions/SourceInlineContentWithPreview/SourceInlineContentWithPreview.ts
@@ -134,7 +134,18 @@ export const SourceInlineContentWithPreviewExtension = createExtension(
           signal,
         });
 
-        const handleBlur = () => store.setState({ selected: undefined });
+        function handleBlur(event: FocusEvent) {
+          // Keep the popup open while selecting error text or returning to the source.
+          if (
+            event.relatedTarget === dom ||
+            (event.relatedTarget instanceof Element &&
+              dom.contains(event.relatedTarget) &&
+              event.relatedTarget.closest(".bn-code-block-source-error"))
+          ) {
+            return;
+          }
+          store.setState({ selected: undefined });
+        }
         dom.addEventListener("blur", handleBlur, { capture: true, signal });
       },
     };
diff --git a/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx b/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
index 65be1cfa13..02084d814f 100644
--- a/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
+++ b/packages/react/src/blocks/SourceWithPreview/SourceWithPreview.tsx
@@ -238,6 +238,9 @@ export const SourceWithPreview = (
           className="bn-code-block-source-error"
           contentEditable={false}
           style={{ display: error ? "block" : "none" }}
+          // Let the browser focus the non-editable error so ProseMirror doesn't
+          // replace the browser's text selection with a source selection.
+          tabIndex={error ? -1 : undefined}
           // 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..1a9ec069ec
--- /dev/null
+++ b/tests/src/end-to-end/sourcepreview/errorSelection.test.tsx
@@ -0,0 +1,138 @@
+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).
+    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(document.activeElement).toBe(error);
+      expect(window.getSelection()?.toString()).toBe(word);
+      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}}`);
+    }
+
+    // Focus leaving the error 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);
+    expect(document.activeElement).toBe(error);
+
+    // Moving directly back from the error to the source must keep it editable.
+    const source = document.querySelector<HTMLElement>(
+      ".bn-source-block-popup code",
+    )!;
+    await userEvent.click(source);
+    const originalSource = source.textContent;
+    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");
+  },
+);