This code re-adds information only to the first block:
window.onload = function(e) {
document.querySelectorAll(".codemirror-textarea").forEach(el => {
const [output] = document.querySelectorAll(".codemirror-textarea");
const editor = CodeMirror.fromTextArea(output, {lineNumbers: true, readOnly: true});
});}
You are looping over the elements, but applies CodeMirror to the first element each time, use el in your loop.
window.onload = function(e) {
document.querySelectorAll(".codemirror-textarea").forEach(el => {
const editor = CodeMirror.fromTextArea(el, {lineNumbers: true, readOnly: true});
});
}