This sample demonstrates Flutter Linux multi-window and popup behavior using native GTK APIs through Dart FFI. It is intended to exercise both GTK3 and GTK4 runners, including API branches needed because GTK4 changed several windowing APIs around its post-Wayland model.
The popup path has been validated with both GTK3 and GTK4 native APIs. The GTK variant check is intentionally local to this demo; Flutter should eventually expose a better Linux GTK variant/version API for app and package code.
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