Bug 50417 – Avoid repainting compositing layers unnecessarily when layout changes

• Status: NEW
• Component: Layout and Rendering

[Simon Fraser (smfr)]

RenderBoxModelObject::styleWillChange() has this code:

        if (diff == StyleDifferenceLayout) {
            // When a layout hint happens, we go ahead and do a repaint of the layer, since the layer could
            // end up being destroyed.
            if (hasLayer()) {
                if (style()->position() != newStyle->position() ||
                    style()->zIndex() != newStyle->zIndex() ||
                    style()->hasAutoZIndex() != newStyle->hasAutoZIndex() ||
                    !(style()->clip() == newStyle->clip()) ||
                    style()->hasClip() != newStyle->hasClip() ||
                    style()->opacity() != newStyle->opacity() ||
                    style()->transform() != newStyle->transform())
                layer()->repaintIncludingDescendants();

This can cause extra repainting of compositing layers when they don't need to be repainted.

[Simon Fraser (smfr)]

<rdar://problem/8863549>

[Simon Fraser (smfr)]

There's also this:


inline void RenderObject::setNeedsLayout(bool b, bool markParents)
{
    bool alreadyNeededLayout = m_needsLayout;
    m_needsLayout = b;
    if (b) {
        ASSERT(!isSetNeedsLayoutForbidden());
        if (!alreadyNeededLayout) {
...
            if (hasLayer())
                setLayerNeedsFullRepaint();

[Simon Fraser (smfr)]

One specific example we should optimize is to avoid repaints when doing a "positioned movement only" layout.