Saxon

So, with Saxon 11 you get a new, blank line after each output of a single xsl:message instruction? Or new lines inserted between the several lines of a single xsl:message?

The StandardMessageHandler writes by default to a Writer that sends messages to the configured Logger, and the default Logger is adding a newline to each message. But the StandardMessageHandler is adding a newline as well, because it doesn't know that the Logger is adding one. Solution not obvious.

Simplest solution is for the standard message handler to refrain from adding the extra newline; if output is to a Logger then you'll get the newline, if it is to a user-supplied Writer destination, then the writer can decide whether to add one or not.