Hi,
This is a known limitation with WPF and we are not aware of any simple workaround for this issue. You can search for "WPF airspace" online and you should be able to find plenty of information and possible workaround on this issue.
The root of the problem is a regular WPF control CAN NOT render over a child window. Most WPF controls are lightweight windowless controls so they are rendered as part of the containing Window object, which manages z-index among all child controls. However in case a child control has its own window handle, it "carves" out a portion of the parent Window object's surface and declare that portion as his own "airspace", which child controls of the parent Window object can no longer occupy. This in turn makes it impossible for the "regular" windowless child controls to be rendered above the windowed child control.
EO.WebBrowser is based on Chromium browser engine which does use its own window handle, so it triggers this issue. We are not aware how CefSharp can workaround this issue since it's based on the same Chromium browser engine, so it should be subjected to the same limitation. However if you can send us a sample project we will be very happy to look into it and see what we can find. You can send files to us through our contact us page:
https://www.essentialobjects.com/contactPlease do not include CefSharp binaries since we can download it directly.
Thanks!