To be more precise about this topic :
When a developer designs a filter, he should also try to develop a corresponding ‘filter preview’ so that the rendering of the preview and the filter match. Sometimes it’s really easy (the same code for the final rendering and the preview make the match), sometimes it’s harder (can be nearly impossible when the filter uses both local and global image information).
In your case, it seems Samj used a somehow wrong default zoom factor for the preview (global view), instead of a local view. I’ll try to fix that by setting a better default preview zoom factor (1:1 here probably).
I don’t claim this will entirely fix the issue, only Samj could tell in this case 
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