Padding and border on the collapsing target lead to snapping on show-transition start or hide-transition end.

[ghost]

On the transition calcSize() is interpreted as the target's inner height/width which is just valid if there's neither a border nor a padding.

Also since .core-collapse-closed is not displayed, the margin, padding, border should be covered in the transition too.

[justinfreitag-zz]

From core-collapse.html

core-collapse adjusts the height/width of the collapsible element to show/hide the content. So avoid putting padding/margin/border on the collapsible directly, and instead put a div inside and style that.

The component plays nice if you follow this