[ck2]

It's because browsers render in memory and then periodically update the video display with a "snapshot" copy. Typically this method is fine for the regular content of an average webpage.

But with zeroTimeout, the movement is happening so fast that the snapshot copy is behind. The browser might be only updating the video every 20ms at the fastest, while the block is moving several pixels every few ms.

But be assured the render of the block is completely happening and then being erased. It's a proper render. If it wasn't, you'd get a permanent "smear" of the results in the browser.