Add warning when leaving form and changes would be lost

Good suggestion! I will convert this to "Feature request", as this is not implemented yet.

At DrupalCon Barcelona 2024 there was also a question regarding "lost" changes and the ability to store them as "Draft". Probably this can be added here as another task not only prevent from leaving the page / closing the sidebar but also suggest to save the changes as "Draft" revision in case content moderation is enabled.

While working on this issue https://www.drupal.org/project/frontend_editing/issues/3478211 ✨ Feature: Detachable Editor Needs work I found that "All entity preview" module can be used widely with frontend editing. I found that if this module is enabled the preview saves the data into private temp storage of Drupal. If the sidebar opens with uuid parameter of the entity as query parameter, then the form loads the input that is stored in the private temp store. This is one of the ways the data loss can be prevented.

Of course the warning will be added, just wanted to share this finding and strongly recommend using https://www.drupal.org/project/all_entity_preview → module in combination with Frontend Editing as it also adds nice preview of the changes that you are doing in the form and you see them right away on the page.

Tested, and looks fine, its doing the job without any bugs.

Thank you for implementing this new functionality.