Saxon

The code is working as designed.

We try to avoid having two trace points for the same (internal) expression. Since the source XSLT

      <xsl:variable name="shorturl" select="substring-after($url, 'http:/')"/>
      <xsl:variable name="protocol" select="'HTTP'"/>
      <external where="{$shorturl}" protocol="{$protocol}"/>

compiles to a single internal expression (a LetExpression, which is subsequently optimized by variable inlining - but the optimization isn't relevant here) it is already traced and so we avoid wrapping another trace instruction around it to represent the xsl:when. Perhaps not perfect, but I think it works, and I'm reluctant to add extra trace points which could upset the people who implement interactive debuggers based on the existing logic.