Hi, thanks for your response.
We're using Domino 9.0.1 FP5
ExtLib 9.0.1.v00_15_20151016-0827
Embedded browser in client and IE 9, IE 10, IE 11, Chrome, Firefox
It's a web issue and the theme we're using is OneUI V3.0.2.
We've had this problem since version 8.5 though, so I don't think it's a new type of behaviour.
Thanks