Function regChange

static/editor.md/lib/codemirror/lib/codemirror.js:3132–3193

(cm, from, to, lendiff)

  // lines are divided into visual lines. regLineChange (below)
  // registers single-line changes.
  function regChange(cm, from, to, lendiff) {
    if (from == null) from = cm.doc.first;
    if (to == null) to = cm.doc.first + cm.doc.size;
    if (!lendiff) lendiff = 0;

    var display = cm.display;
    if (lendiff && to < display.viewTo &&
        (display.updateLineNumbers == null || display.updateLineNumbers > from))
      display.updateLineNumbers = from;

    cm.curOp.viewChanged = true;

    if (from >= display.viewTo) { // Change after
      if (sawCollapsedSpans && visualLineNo(cm.doc, from) < display.viewTo)
        resetView(cm);
    } else if (to <= display.viewFrom) { // Change before
      if (sawCollapsedSpans && visualLineEndNo(cm.doc, to + lendiff) > display.viewFrom) {
        resetView(cm);
      } else {
        display.viewFrom += lendiff;
        display.viewTo += lendiff;
      }
    } else if (from <= display.viewFrom && to >= display.viewTo) { // Full overlap
      resetView(cm);
    } else if (from <= display.viewFrom) { // Top overlap
      var cut = viewCuttingPoint(cm, to, to + lendiff, 1);
      if (cut) {
        display.view = display.view.slice(cut.index);
        display.viewFrom = cut.lineN;
        display.viewTo += lendiff;
      } else {
        resetView(cm);
      }
    } else if (to >= display.viewTo) { // Bottom overlap
      var cut = viewCuttingPoint(cm, from, from, -1);
      if (cut) {
        display.view = display.view.slice(0, cut.index);
        display.viewTo = cut.lineN;
      } else {
        resetView(cm);
      }
    } else { // Gap in the middle
      var cutTop = viewCuttingPoint(cm, from, from, -1);
      var cutBot = viewCuttingPoint(cm, to, to + lendiff, 1);
      if (cutTop && cutBot) {
        display.view = display.view.slice(0, cutTop.index)
          .concat(buildViewArray(cm, cutTop.lineN, cutBot.lineN))
          .concat(display.view.slice(cutBot.index));
        display.viewTo += lendiff;
      } else {
        resetView(cm);
      }
    }

    var ext = display.externalMeasured;
    if (ext) {
      if (to < ext.lineN)
        ext.lineN += lendiff;