I have a grid with a filter row in the top. When zooming in, borders disappear.
I could also repro it with your own demos of the combobox: http://demos.telerik.com/kendo-ui/combobox/index. When I zoom in to 90%, I see a missing top border for t-shirt size (see attached). When I zoom in further other borders disappear and appear.
I use Chrome Version 62.0.3202.94