Topic on Extension talk:RevisionSlider

Steps to reproduce:

1. Go to https://www.mediawiki.org/w/index.php?diff=2285787&oldid=2285785&title=MediaWiki&diffmode=source
2. Move the mouse pointer to the yellow line (or bar) right before the yellow knob
3. Move the mouse pointer to the yellow knob
4. Move the mouse pointer to the blue knob
5. Move it to the blue line
6. Move it to the empty area between the blue line and the slider button
7. (Optional) Move the mouse pointer back to revision bars area.

Expected:

No console errors

Actual:

Uncaught TypeError: Cannot read property 'top' of undefined$.extend.showTooltipsOnMouseMoveHandler @ VM156:formatted:5849(anonymous function) @ VM156:formatted:5803jQuery.event.dispatch @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:65elemData.handle @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin…:60

This doesn't seem to cause have any immediate problem. It was simply something noticed while testing out the interface.

Simpler reproduction: repeatedly hover over the blue knob, the blue line, and the white space right in front of it.