You can resize and move an undecorated window using glfwGetWindowSize and glfwSetWindowPos. These can be used alongside your own UI to provide drag and resizing. For an example of this see the Dear ImGui Docking branch with viewports.
One problem with this is that you no longer receive OS resizing events such as the Windows WIN+LEFT for tiling the window on a part of the screen.
I am not sure of the status of the linked issue, but it appears the fork mentioned no longer exists. A newer version appears to be in progress here: