Hi Pietro,
Wow, 6 years Welcome to the new Freeter! And thanks for the feedback!
it opens a new window that can never go behind the Freeter main window. Is it an expected behavior?
Expected Previously Freeter opened "new tab" links in the same webpage widget, just like common links. However there were scenarios when that caused some issues. That's why I decided to add the new windows. The idea was to open them as child windows, so that when you switch to another app and then switch back to Freeter the child windows do not get lost. This also helps to not forget to close the unneeded windows.
I will need to think more about it. Maybe you have other ideas on what else could be added or changed to make the new windows more usable in your workflow?