diff --git a/src/components/dialogs/algorithm-dialog.tsx b/src/components/dialogs/algorithm-dialog.tsx
index 6259bba..50d6a47 100644
--- a/src/components/dialogs/algorithm-dialog.tsx
+++ b/src/components/dialogs/algorithm-dialog.tsx
@@ -34,15 +34,13 @@ export function AlgorithmDialog({ adding, onClose }: { adding: boolean; onClose:
         if (cancelled) return;
         instance = createCodeEditor(host.current!);
         editor.current = instance;
-        instance.setValue(
-          adding
-            ? ""
-            : getFunctionBody(
-                algorithm === algorithms[sort as keyof typeof algorithms]
-                  ? sources[sort as keyof typeof sources]
-                  : algorithm,
-              ),
-        );
+        const builtin = algorithm === algorithms[sort as keyof typeof algorithms];
+        const body = adding
+          ? ""
+          : getFunctionBody(builtin ? sources[sort as keyof typeof sources] : algorithm);
+        // Built-ins use two-space function indentation. Preserve custom source verbatim,
+        // since changing whitespace inside a template literal can change its value.
+        instance.setValue(builtin ? body.replace(/^ {2}/gm, "").trim() : body);
         instance.clearSelection();
         setStatus("ready");
       })
diff --git a/test/browser/site.spec.mjs b/test/browser/site.spec.mjs
index e4b8999..b7e7c86 100644
--- a/test/browser/site.spec.mjs
+++ b/test/browser/site.spec.mjs
@@ -510,6 +510,11 @@ test("all built-ins can be edited and saved from readable production source", as
       .locator("[data-panel=sort-algorithm] .js-editor.ace_editor")
       .evaluate((element) => globalThis.ace.edit(element).getValue());
     expect(source).toContain("AS.");
+    expect(source).toBe(source.trim());
+    if (id === "bubble") {
+      expect(source).toContain("\nlet swapped;");
+      expect(source).toContain("\n  swapped = false;");
+    }
     await page.locator("#save-algorithm-edit").click();
     await expect(page.locator("#modal-sort")).toBeHidden();
     await page.locator(`#sort-options [data-sort="${id}"]`).click();