On top of a set of 8 maximised windows I want to display a new window,
like a popup, to write a letter about the info in the topmost window.

The new window opens maximised, when it is changed to smaller a size,
all other windows resize as well. It looks like a mess.

I experimented with selecting windows that are already open, or opening
a new window, but I didn't found a solution yet.

Is there a way to get this done?