This an example of a custom block attribute added only for the core/html block (Custom HTML) with a simple input field to verify it will be correctly saved in the block widget screen.
It is a base to test the issue in the Gutenberg project.
Simply put in your wp-content/mu-plugins/ folder of your WordPress instance:
- the
gutenberg-core-html-block-attribute-widget-test.phpfile - the
dist/folder
You can then see the Custom attributes panel with a custom attribute field when a Custom HTML block is selected in the screen.
- Enter something in the
A custom attributefield of a Custom HTML block in the block widget screen - Save the block widget screen
- Refresh the block widget screen and you should see in the
A custom attributefield the value previously entered
- Add a Custom HTML in a empty widget area and enter a content in the block - a simple text for example
- Enter
A custom attributein the corresponding block attribute - Save the widget screen
- Refresh the widget screen
- See that the
A custom attributefield is again empty as at the beginning.