Add expandAll/collapseAll to diffs

[atamarzban]

Summary

• Adds programmatic expandAll() and collapseAll() methods to FileDiff, VirtualizedFileDiff, and the React useFileDiffInstance hook for controlling collapsed "N unmodified lines" sections
• Adds a text toggle button ("Expand all" / "Collapse all") in the file header next to the filename — visible by default, opt-out via disableExpandAllButton: true
• Button is automatically hidden when expandUnchanged or disableFileHeader is set

API

Vanilla JS:

const diff = new FileDiff(options);
diff.expandAll();   // expand all collapsed sections
diff.collapseAll(); // collapse everything back

React:

const { ref, expandAll, collapseAll } = useFileDiffInstance(options);

Disable the built-in button:

const diff = new FileDiff({ disableExpandAllButton: true, ...rest });

🤖 Generated with Claude Code

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[atamarzban]

@amadeus Would appreciate a review! 🙏

[amadeus]

Question on this -- shouldn't you be able to accomplish this functionality already with toggling the expandUnchanged prop? Or is there something about that API that doesn't work that I'm not seeing.

It's not the cleanest, but right now in the VanillaJS api you could accomplish it with:

const diff = new FileDiff(options);
diff.setOptions({ ...diff.options, expandUnchanged: true })
diff.setOptions({ ...diff.options, expandUnchanged: false })

// and in react, you would pass in the prop change to options:

<FileDiff
  // ...
  options={{ expandUnchanged: true }}
/>
<FileDiff
  // ...
  options={{ expandUnchanged: false }}
/>

[amadeus]

Also, sorry for the delays on this, been traveling a bunch lately so catching up when i can.

[atamarzban]

@amadeus thx for the taking a look! setOptions with expandUnchanged does cover the basic expand/collapse toggle. The main thing this PR adds is the built-in header button (next to the filename, similar to GitHub). Since it lives inside the shadow DOM, it’s not something consumers can easily wire up themselves, so having it built-in felt worthwhile.

There’s also a subtle behavioral difference: collapseAll() clears individually-expanded hunks too, so if someone manually expanded a few sections and then hits “Collapse all”, they get a clean reset. Toggling expandUnchanged back to false would leave those individual expansions intact.

That said, the button doesn’t strictly need the new imperative API. It could toggle expandUnchanged internally. The separate methods just make the wiring cleaner. Happy to simplify if you’d prefer a different approach.