Function updateDisplayIfNeeded

static/plugins/codemirror/codemirror.js:4106–4176

(cm, update)

  // (returning false) when there is nothing to be done and forced is
  // false.
  function updateDisplayIfNeeded(cm, update) {
    var display = cm.display, doc = cm.doc;

    if (update.editorIsHidden) {
      resetView(cm);
      return false
    }

    // Bail out if the visible area is already rendered and nothing changed.
    if (!update.force &&
        update.visible.from >= display.viewFrom && update.visible.to <= display.viewTo &&
        (display.updateLineNumbers == null || display.updateLineNumbers >= display.viewTo) &&
        display.renderedView == display.view && countDirtyView(cm) == 0)
      { return false }

    if (maybeUpdateLineNumberWidth(cm)) {
      resetView(cm);
      update.dims = getDimensions(cm);
    }

    // Compute a suitable new viewport (from & to)
    var end = doc.first + doc.size;
    var from = Math.max(update.visible.from - cm.options.viewportMargin, doc.first);
    var to = Math.min(end, update.visible.to + cm.options.viewportMargin);
    if (display.viewFrom < from && from - display.viewFrom < 20) { from = Math.max(doc.first, display.viewFrom); }
    if (display.viewTo > to && display.viewTo - to < 20) { to = Math.min(end, display.viewTo); }
    if (sawCollapsedSpans) {
      from = visualLineNo(cm.doc, from);
      to = visualLineEndNo(cm.doc, to);
    }

    var different = from != display.viewFrom || to != display.viewTo ||
      display.lastWrapHeight != update.wrapperHeight || display.lastWrapWidth != update.wrapperWidth;
    adjustView(cm, from, to);

    display.viewOffset = heightAtLine(getLine(cm.doc, display.viewFrom));
    // Position the mover div to align with the current scroll position
    cm.display.mover.style.top = display.viewOffset + "px";

    var toUpdate = countDirtyView(cm);
    if (!different && toUpdate == 0 && !update.force && display.renderedView == display.view &&
        (display.updateLineNumbers == null || display.updateLineNumbers >= display.viewTo))
      { return false }

    // For big changes, we hide the enclosing element during the
    // update, since that speeds up the operations on most browsers.
    var selSnapshot = selectionSnapshot(cm);
    if (toUpdate > 4) { display.lineDiv.style.display = "none"; }
    patchDisplay(cm, display.updateLineNumbers, update.dims);
    if (toUpdate > 4) { display.lineDiv.style.display = ""; }
    display.renderedView = display.view;
    // There might have been a widget with a focused element that got
    // hidden or updated, if so re-focus it.
    restoreSelection(selSnapshot);

    // Prevent selection and cursors from interfering with the scroll
    // width and height.
    removeChildren(display.cursorDiv);