Keep newlines when copying source lines

Fixes 956f736acc

src/lib/syntax.ts

@@ -156,12 +156,17 @@ export function lineNumbersGutter(tree: Root) {
           });
         }
 
-        // Add a line, and the eol.
-        lineNumber += 1;
-        replacement.push(createLine(line, lineNumber), {
-          type: "text",
-          value: match[0],
+        // Add a line, with its eol inside the line's content so that copying
+        // the line keeps it. The eol is wrapped in a span because the HTML
+        // parser drops a newline that directly follows a `<pre>` start tag.
+        line.push({
+          type: "element",
+          tagName: "span",
+          properties: {},
+          children: [{ type: "text", value: match[0] }],
         });
+        lineNumber += 1;
+        replacement.push(createLine(line, lineNumber));
 
         start = index + 1;
         textStart = match.index + match[0].length;

tests/e2e/clipboard.spec.ts

@@ -87,6 +87,22 @@ test("copy to clipboard", async ({ page, browserName, context }) => {
     await expectClipboard("Inline `code` has `back-ticks around` it.\n");
   }
 
+  // Selected source lines keep their newlines, including an empty line.
+  {
+    await page.goto(`${markdownUrl}/tree/main/markdown-code.md#L1`);
+    const from = await page.locator("#L5").getByText("```").boundingBox();
+    const to = await page.locator("#L8").getByText("plain:").boundingBox();
+    if (!from || !to) {
+      throw new Error("source lines not rendered");
+    }
+    await page.mouse.move(from.x, from.y + from.height / 2);
+    await page.mouse.down();
+    await page.mouse.move(to.x, to.y + to.height / 2, { steps: 5 });
+    await page.mouse.up();
+    await page.keyboard.press("ControlOrMeta+C");
+    await expectClipboard("```\n\n```\n");
+  }
+
   // Link to a patch comment.
   {
     await page.goto(`${cobUrl}/patches`);