Created attachment 368873 [details]
mockup

In line with bug #756060, to create additional space savings and efficiency, I would recommend that we remove the big "Test URL..." button on the right of the URL GtkEntry, and replace it by a GtkEntry "secondary icon". The button is huge (especially in non-English locales), taking away space that could be used for the URL itself.

My recommendation for its replacement is to set these properties onto the URL GtkEntry:

secondary-icon-name = "go-jump" (this is the recommended way as per https://developer.gnome.org/gtk3/stable/gtk3-Stock-Items.html#GTK-STOCK-JUMP-TO:CAPS ...)

secondary-icon-tooltip-text = "Click to test this link by opening it in your web browser" (translatable of course)

And then connecting to the secondary-icon's activated signal like you would with a normal button.

Attaching a simulation of what it would look (and behave) like, done with the Gtk Inspector.