Implement the configure callback in the zxdg_toplevel_v6_listener. Code is in Tools/wpe/backends/WindowViewBackend.cpp For reference, cog implements it already (https://github.com/Igalia/cog), so the job is basically porting the logic to MiniBrowser.
Support for the .configure callback has been implemented in bug #247176 *** This bug has been marked as a duplicate of bug 247176 ***