Steps to Reproduce:
Go to edit mode in a module, https://ru.wikipedia.org/wiki/Модуль:Песочница/Carn/Frame for example.
Place the top edge of the editor textbox including buttons near the top of the browser window.
Activate CodeEditor with a button <> in the left top corner, so line numbers appear.
Move text cursor around a bit with keyboard arrows.
Actual Results: the whole page begins to jump up and down a bit.
Expected Results: the page should not twitch jerkily.