The Test Pilot team has opted not to continue with approach to the experiment. Tentatively we are working on another approach in the tab-split-2 repo
Tab Split is a Firefox Test Pilot experiment which allows you to see two tabs in the same window, side by side.
Currently, Tab Split uses an Embedded WebExtension API Experiment. There is a bug which prevents us from using any WebExtension apis, so the implementation just calls into the API Experiment from the WebExtension and does everything using XPCOM and XUL code.
To test, run npm run build, then load Firefox Nightly, visit about:debugging, click Load Temporary Add-on, and navigate to tab-split/webext/manifest.json.
To iteratively develop, you can run npm run watch and press the reload button in about:debugging after making any changes.
The checklist of acceptance criteria for this project is located in the documentation.