Function updateDisplayIfNeeded

external/.scrollLibs.js:5904–5987

(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"