fix(gutenberg): prevent setAttributes on initial mount to avoid dirtying blocks

[dfross]

Summary

In packages/gutenberg/src/index.tsx, an initial render lifecycle bug causes clean posts, templates, and template parts to be marked as modified ("Review changes") immediately upon opening the WordPress Site Editor or Block Editor.

Problem

The component initializes internal state with empty arrays:

const [classNames, setClassNames] = useState({ active: [], dynamic: [] });

A useEffect hook depends on [classNames] and synchronizes it to Gutenberg via props.setAttributes(...):

useEffect(() => {
    props.setAttributes({
        className: [...classNames.active, ...classNames.dynamic].join(" "),
    });
}, [classNames]);

Because classNames starts empty, this effect fires on initial mount and writes { className: "" } to the block attributes before the saved classes finish loading. Calling setAttributes() during initial render mutates Gutenberg's in-memory entity store, immediately flagging clean pages and template parts with unsaved changes.

Solution

Added an isInitialMount ref guard (useRef(true)):

• On initial render, the effect detects the mount phase, sets isInitialMount.current = false, and returns without calling setAttributes().
• Subsequent user interactions (hovering over classes for dynamic preview, toggling classes) continue to function normally.
• Prevents Gutenberg blocks from being wiped with className: "" on initial load.