tuned error -- allow user to click on hyperlink to open remote file;

no comment -- File.read_stream already enforces UTF8 (like HTML5) and HTTP servers often produce a wrong charset encoding;

improved support for external URLs, based on standard Java network operations;

basic URL operations (with Isabelle/Scala error handling);

proper Args.name vs. Args.text as documented (in contrast to adhoc union in 75aaee32893d, which had to cope with more limited Args.T);

allow text cartouches in regular outer syntax categories "text" and "altstring";

tuned comments (see Scala version);

dismiss popups more carefully (amending 7e8c11011fdf), notably allow mouse dragging within some tooltip, e.g. for text selection;

clarifed results range;

more explicit message discrimination;

avoid confusion about pointless cursor movement with external links;

prefer regular Goal_Display.pretty_goals, without censorship of options;