Function posFromMouse

static/editor.md/lib/codemirror/lib/codemirror.js:3424–3438

(cm, e, liberal, forRect)

  // selections, and tries to estimate a character position even for
  // coordinates beyond the right of the text.
  function posFromMouse(cm, e, liberal, forRect) {
    var display = cm.display;
    if (!liberal && e_target(e).getAttribute("cm-not-content") == "true") return null;

    var x, y, space = display.lineSpace.getBoundingClientRect();
    // Fails unpredictably on IE[67] when mouse is dragged around quickly.
    try { x = e.clientX - space.left; y = e.clientY - space.top; }
    catch (e) { return null; }
    var coords = coordsChar(cm, x, y), line;
    if (forRect && coords.xRel == 1 && (line = getLine(cm.doc, coords.line).text).length == coords.ch) {
      var colDiff = countColumn(line, line.length, cm.options.tabSize) - line.length;
      coords = Pos(coords.line, Math.max(0, Math.round((x - paddingH(cm.display).left) / charWidth(cm.display)) - colDiff));
    }
    return coords;
  }

  // A mouse down can be a single click, double click, triple click,
  // start of selection drag, start of text drag, new cursor