IE performance - border-radius styles in iceblue

Steps to Reproduce

<zk>
	<zscript><![CDATA[
		ListModelList model = new ListModelList(Collections.nCopies(1000, "dummy"));
	]]></zscript>

	<panel width="100%" height="100%">
	<panelchildren>

	<grid model="${model}" height="700px">
		<custom-attributes org.zkoss.zul.grid.rod="true"/>
		<template name="model">
			<row>
				<cell>Cell</cell>
				<cell>Cell</cell><cell>Cell</cell>
			</row>
		</template>
	</grid>

	</panelchildren>
	</panel>
</zk>

open with IE11 and Iceblue theme

Current Result

CPU/GPU usage increase drastically making the page almost unresponsive

Debug Info

classic themes such as "breeze" don't have this problem

Root Cause

The border-radius styles of the <panel> component cause the performance hit on IE11

The negative effect gets worse in this case due to the gif loading animation ZK-4082

Workaround

1) A <panel> without a border can just be replaced with a simple <div> component or no component at all.

2) Disable the border radius for <panel> without a border

	.z-panel-noborder {
		border-radius: 0;
	}