WebBrowser.Visible property will not hide/show the control, see MSDN documentation below:
Sets or gets a value that indicates whether the object is visible or hidden.
When the Windows Internet Explorer application is first created, it is hidden. It becomes visible after the Navigate method or the GoSearch method is used.
This method only applies when you are automating Internet Explorer. You co-create an instance of Internet Explorer, and then set this method to TRUE to show the instance.
The WebBrowser object saves the value of this property, but otherwise ignores it.
You can add a Panel control from “Form Designer” – Standard/Enhanced tab then add EmbeddedWB control on top of the panel. Finally, hide/show panel control.
Maindlg.panel.visible = false