Bug 126460

In each RenderSelectionInfo::repaint() it's used the RenderObject::containerForRepaint(). Which in most of the situations returns the RenderNamedFlowThread for all the elements under RenderNamedFlowThread in the render tree. This causes that for every element under RenderNamedFlowThread the method RenderFlowThread::repaintRectangleInRegions() is called. Taking a look to this method it has a loop over all the regions forcing a repaint. This means that if you have 1000 regions, even if you're just selecting in one of them, a repaint in the rest of regions is executed: void RenderFlowThread::repaintRectangleInRegions(const LayoutRect& repaintRect, bool immediate) const { if (!shouldRepaint(repaintRect) || !hasValidRegionInfo()) return; LayoutStateDisabler layoutStateDisabler(&view()); // We can't use layout state to repaint, since the regions are somewhere else. // We can't use currentFlowThread as it is possible to have interleaved flow threads and the wrong one could be used. // Let each region figure out the proper enclosing flow thread. CurrentRenderFlowThreadDisabler disabler(&view()); for (auto iter = m_regionList.begin(), end = m_regionList.end(); iter != end; ++iter) { RenderRegion* region = *iter; region->repaintFlowThreadContent(repaintRect, immediate); } } This could be avoided but just issuing a repaint for the regions affected by the repaintRect.