After trying to change the pattern, the base image window is vibrating. Moving the selector around, changing the tiling pattern, changing rotation, changes of any kind don’t seem to make a difference. Itseems that the image is stuck trying to change its size. If I change the image to one from the demo gallery, the vibrating stops and does not restart when changing the tiling. If I choose the uploaded image again, the first time I try to change the tiling, it starts vibrating again. That image is a jpg 896 × 1152px. Changing to an uploaded 260x260px png image, no vibration. I rotated the vibrating image 90 degrees and re-imported it into Repper. No vibrating. I saved the original image and imported it to see if just that act of saving it made a difference, it did not. The re-exported orginal image still vibrates, as does any 896 × 1152px I import. 1152 × 896px images do not vibrate.
Thanks for filing this issue!
I think I know the cause and it should be easy to fix. The problem is something to do with the scrollbar, which will show if there is a need for scrolling, but it appearing will change the size of the edit controls, which will get a little smaller and so no longer a scrollbar is needed. It will go back and forth between those two 
The quick fix for now is easy: slightly resize your browser window.
In the meantime, I will fix this so it won’t ever happen.
This has happened to me too, but also in the original app. It’s random, and it will happen whenever you choose the pattern in that photo. The only way to avoid it is to delete the image and upload it again. But remember to export your screenshots before, or you’ll lose them (ask me how I know, LOL).
I’ll try to resize the browser window if it happens again. That seems like an easy enough fix