Is it possible to create and manage multiple windows (parent, child) with GLFW? One window with opengl and the rest differently (e.g. with traditional GUI)?
Welcome to the GLFW forum.
Yes, GLFW can create and manage multiple windows, though parent / child window relationships need to be set using the native access API.
GLFW doesn’t expose traditional GUI rendering, so you will need to use the native access API linked to above.
Note that an alternative is to use a UI library such as Dear ImGui which has a GLFW backend and can handle multiple windows using the docking branch with external viewports:
This is the solution I personally use for a 3D editor with tool windows.