If the article editing bar doesn't completely fit when the article is opened (for example for files from commons or global user pages) the whole bar first jumps down into the article content area, does a bit of movement and jumps back into its position. Also, that makes the content make a visible jump.
See the following gif for an example. I've refreshed the page when the movement started.
Browser: Safari 11.0.2, Chrome 63, Firefox 58
OS: macOS High Sierra