Fix heading auto-link hijacking nested link labels (#668)

src/Markdig/Syntax/LinkReferenceDefinition.cs

@@ -136,6 +136,21 @@ public class LinkReferenceDefinition : LeafBlock
     /// </remarks>
     public CreateLinkInlineDelegate? CreateLinkInline { get; set; }
 
+    /// <summary>
+    /// Gets a value indicating whether this reference definition is allowed to resolve into a link when its
+    /// label is nested inside another link or image bracket that is still open (and may still become a valid
+    /// link/image once fully parsed).
+    /// </summary>
+    /// <remarks>
+    /// This is <c>true</c> for regular, user-authored reference definitions so that CommonMark's "links cannot
+    /// contain links" resolution order is preserved. Implicitly generated reference definitions (e.g. the
+    /// automatic heading anchors created by the auto-identifier extension) override this to <c>false</c> so
+    /// they don't silently hijack a bracket pair that is part of a larger, still-open link label, such as
+    /// <c>[Some text [HeadingText]](https://example.com)</c> (see
+    /// https://github.com/xoofx/markdig/issues/668).
+    /// </remarks>
+    internal virtual bool AllowResolutionInsideOpenLink => true;
+
     /// <summary>
     /// Tries to the parse the specified text into a definition.
     /// </summary>