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.
We recently upgrade Kendo UI from version v2019.1.220 to 2019.2.619 and our user base is freaking out because they can't select the text on web pages that use the splitter.