Use a single display-safe viewport owner, responsive Flex sections, and the warm Ebox Flex reference visual language across the Showcase and compact examples.\n\nVerification: make check EMACS=/Applications/Emacs.app/Contents/MacOS/Emacs; GUI single-window screenshots for compact/fullscreen Overview, Work items interaction, dark Theme, core, and optional UI views.
Add a public interactive example covering Grid layout, layered canvas and panel backgrounds, Context-derived themes, reactive state, in-memory Data Controller updates, keyed rows, navigation, and event dispatch. Defer one viewport sync after display so fullscreen frames render the actual visible dimensions.
Provide a runnable ETAF-only example and an optional UI catalog entry point with lifecycle-safe open/close commands, tests, and bilingual README documentation.
