Scroll position is not preserved when custom markdown editor was closed and opened again

[sandy081]

Testing #138040

• Open a markdown file as custom markdown preview editor
• Scroll down
• Close the editor and open again
  🐛 Scroll position is not retained

[babakks]

@mjbvz I'd like to fix it. I appreciate any hints.

[rzhao271]

It works, though I notice that after reopening, if I try scrolling immediately, it seems to take some time to register the scroll.