See Kendo UI v2019.1.220 Distributed Source Code module kendo.splitter.js _resize() method (Lines 285-240)
1) The splitter _resize method iuses jQuery width() and height() methods to set pane sizes.
2) These methods get/set the clientHeight and clientWidth of the pane element rather than the offsetWidth and offsetHeight.
3) If any pane has a border or padding, the last pane will extend beyond the limits of the splitter element.
4) Recommend using offsetWidth and offsetHeight to get/set all pane sizes.
Adding animation on splitter would be better, currently there is no animation on splitter. animation like slide,fade in, fadeout would be fine.
Some kind of animation for the splitters when you collapse/expand them? Even nowadays there are people not used to the web environment, and if the collapsible pane just hides instantly (or almost) they will first think 'Where did my *whatever* went!?' so having some kind of easing they would see where did it go.