[Discussion] Shader works unexpected on TiledBackgrounds.

<?xml version="1.0" encoding="UTF-8" ?>
<c2effect>
	<!-- About -->
	<id>Scroll</id>			<!-- Never change the ID.  Change the name instead -->
	<name>Scroll</name>
	<category>Distortion</category>
	<description>Seamlessly scroll the texture within a given amount of time</description>
	<author>Somebody</author>
	
	<!-- Settings -->
	

	<!-- Extend the bounding box for effect processing by a number of pixels to show the edges
		 of effects which go beyond the object edges, e.g. blur and warp. -->
	<extend-box-horizontal>0</extend-box-horizontal>
	<extend-box-vertical>0</extend-box-vertical>
	
	<!-- Set to true if the background is sampled (samplerBack is referenced at all in the shader) -->
	<blends-background>false</blends-background>
	
	<!-- Set to true if the background is not sampled at 1:1 with the foreground (e.g. the
		 background texture co-ordinates are modified in some way by the shader, as done
		 by Glass and Lens) -->
	<cross-sampling>false</cross-sampling>
	
	<!-- Set to true if the effect changes over time, e.g. Noise or Warp effects. -->
	<animated>true</animated>
	
	<!-- Parameters -->
	<parameters>
		<param>
			<name>X shift</name>
			<description>Amount of time for a full horizontal shift</description>
			<type>float</type>
			<initial>0</initial>
			<uniform>xshift</uniform>
		</param>
		<param>
			<name>Y shift</name>
			<description>Amount of time for a full vertical shift</description>
			<type>float</type>
			<initial>0</initial>
			<uniform>yshift</uniform>
		</param>
	</parameters>
</c2effect>
[/code:3p4wo9m0]
scroll.fx
[code:3p4wo9m0]/////////////////////////////////////////////////////////
// Scroll effect
varying mediump vec2 vTex;
uniform lowp sampler2D samplerFront;
uniform lowp float seconds;

uniform lowp float xshift;
uniform lowp float yshift;

void main(void)
{	
	mediump vec2 tex = vTex;
	
	if (xshift != 0.0) tex.x = mod(tex.x-mod(seconds,xshift)/xshift,1.0);
	if (yshift != 0.0) tex.y = mod(tex.y-mod(seconds,yshift)/yshift,1.0);
		
	gl_FragColor = texture2D(samplerFront,tex);
}[/code:3p4wo9m0]

For some reasons tiledbackgrounds seem to do it though either layout size or window size. Sprites work as expected.
The tiledbackgrounds is 16x16 and its width is a multiple of 16 and its height is 16.
Any support, will be greatly appreciated, in addition can you explain what you done so I can learn. 
Thank you