A text editor can use an array as data structure. You only need it to be fast during the typing, where you are only changing one line. But for entering new lines, the extra latency needed for rebuilding the array after pressing enter is not noticeable for several million lines.
The more challenging part of a text editor is making sure you only render what the user sees.
The more challenging part of a text editor is making sure you only render what the user sees.