re-sizable tiles and/or only hide interface on specific window
When you use the internal tiling of vivaldi, you can not resize the tiles, so except if you have a very big screen, one of the tiles may not display enough content for the use, while the other one might anyway show "too much".
I thought I could workaround this by simply using my window manager (fiy: i3, a tiling wm) and setting the sizes with my window manager. But then, I have the vivaldi's interface twice, which is annoying since I put the tab list on right side. So, this eats a lot the width of my screen for an empty list on the "slave" window.
Using CTRL+F11 on "slave" window does not does what I want, since it hides the interface of both windows, which I do not want: my "master" window's tab list is useful to allow me navigating between tabs (also, someone could say he would use notes, your new notification system or other nice vivaldi's feature on the main window, which would be disabled then).
I wanted to do that stuff when configuring a web thing while reading the doc and testing it: usually, I run doc and tests on a screen while tinkering on the other.
PS: the new forum's interface is confusing, it's hard to see where are writing areas, and by default the main writing area is way too small imho. Seems a bit slow on login, too.