more robust Position.setmp_thread_data, independently of Output.debugging (essentially reverts f9ec18f7c0f6, which was motivated by clean exception_trace, but without transaction positions the Isabelle_Process protocol breaks down);

refrain from forcing a hardwired SHELL value, cf. 1494ded298a6 but it becomes obsolete again in 549969a7f582 and follow-ups;

bad_result: report fully explicit message;

observe additional isabelle-jedit.css for component and user; visial separation of message divs;

added checkboxes for debug/tracing filter; misc tuning;

more abstract view on prover output messages;

added some tooltips;

HTML_Panel.handler as overridable method;

added Library.undefined (in Scala);

more systematic treatment of internal state, which belongs strictly to the main actor, not the Swing thread; do not re-use mutable DOM -- avoid races wrt. the rendering engine; more thorough resize -- always recalculate metrics/margin synchronously; asynchronous setDocument; tuned;