Popup opened from within a Splitter has lower z-index than the resize handle

[dtopalov]

When a Popup is opened from within a Splitter pane, it is rendered below the Splitter resize handle instead of above it because the resize handle element has a z-index of 200, while the Popup by default assumes a z-index calculated from the z-index of its anchor element:

https://stackblitz.com/edit/angular-rbpdcs-cwmgqn?file=app/app.component.ts

(open the column menu of the right Grid column)

When the resize handle z-index is set to 1, the layout is as expected:

https://stackblitz.com/edit/angular-rbpdcs-nwtguy?file=app/app.component.ts

If there is no particular reason for setting the Splitter resize handle to 200, we may consider setting to 1 to avoid similar issues.