Function updateDisplayIfNeeded

static/libs/codemirror.js:4167–4237

(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);