Excel spec

*In a protected sheet

1. auto-fill can fill unlocked cells within a protected sheet
2. if auto-fill destination region contains at least 1 locked cell, it doesn't fill any cell and popup a warning message.
3. a destination region excludes the selected source cells.
   Select A1:B1 and drag to C1:D1, the destination is C1:D1.

related doc

Fill data automatically in worksheet cells

debug info

public abstract class AbstractCellHandler extends AbstractHandler {
	private static final long serialVersionUID = -1020093806000599051L;

	@Override
	public boolean isEnabled(Book book, Sheet sheet) {
		return book != null && sheet != null && ( !sheet.isProtected() ||
				Ranges.range(sheet).getSheetProtection().isFormatCellsAllowed());
	}
}

The super class only enables a handler when format cell is allowed, but AutoFillHandler is not related to cell format.