fullscreen-shell: present surface and sub-surfaces

We are unsure whether we are using the fullscreen-shell as intended and would appreciate your help.

For our project we use the fullscreen-shell as follows: First a main surface is passed using present_surface (ZWP_FULLSCREEN_SHELL_V1_PRESENT_METHOD_DEFAULT). Then, further sub-surfaces are assigned to the main surface. These may also be (partially) outside the main interface. The parts of the sub-surfaces that lie outside the main surface are not displayed on the screen. This is the behavior we want.

To switch off the output we present a null surface. To switch on again, the main surface is presented again. Unfavorably, the displayed section changes. The fullscreen-shell calculates the geometry using surface_subsurfaces_boundingbox(). The position and size of the sub-surfaces are taken into account.

What are we doing wrong? What better way to handle it? Thanks a lot!