hflex + width is no longer supported in ZK 8:

Since the captcha component has a default size of 200px * 50px, which is used for both internal image generation and rendering it is currently impossible to stretch the captcha element using hflex/vflex.

If the component width/height were decoupled from the rendering width/height it would be possible to define the internal image dimensions, separate from the rendering width, height.

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A possible workaround is to clear the width/height at the client side and apply the hflex instead then the widget will resize automatically:

<zk xmlns:w="client">
	<window border="normal" width="500px" sizable="true" height="70px">
		<captcha id="cap" 
			w:onBind="this.setWidth(null); this.setHflex(1); 
					  this.setHeight(null); this.setVflex(1);"/>
		<button label="redraw" onClick="cap.randomValue()"/>
	</window>
</zk>