diff --git a/src/Pure/Admin/build_doc.scala b/src/Pure/Admin/build_doc.scala
--- a/src/Pure/Admin/build_doc.scala
+++ b/src/Pure/Admin/build_doc.scala
@@ -1,102 +1,101 @@
 /*  Title:      Pure/Admin/build_doc.scala
     Author:     Makarius
 
 Build Isabelle documentation.
 */
 
 package isabelle
 
 
 object Build_Doc
 {
   /* build_doc */
 
   def build_doc(
     options: Options,
     progress: Progress = new Progress,
     all_docs: Boolean = false,
     max_jobs: Int = 1,
     docs: List[String] = Nil)
   {
     val store = Sessions.store(options)
 
     val sessions_structure = Sessions.load_structure(options)
     val selected =
       for {
         session <- sessions_structure.build_topological_order
         info = sessions_structure(session)
         if info.groups.contains("doc")
         doc = info.options.string("document_variants")
         if all_docs || docs.contains(doc)
       } yield (doc, session)
 
     val documents = selected.map(_._1)
     val selection = Sessions.Selection(sessions = selected.map(_._2))
 
     docs.filter(doc => !documents.contains(doc)) match {
       case Nil =>
       case bad => error("No documentation session for " + commas_quote(bad))
     }
 
     progress.echo("Build started for sessions " + commas_quote(selection.sessions))
     Build.build(options, selection = selection, progress = progress, max_jobs = max_jobs).ok ||
       error("Build failed")
 
     progress.echo("Build started for documentation " + commas_quote(documents))
     val doc_options =
       options + "document=pdf" + "document_output=~~/doc" + "document_output_sources=false"
     val deps = Sessions.load_structure(doc_options).selection_deps(selection)
 
     val errs =
       Par_List.map((doc_session: (String, String)) =>
         try {
-          progress.expose_interrupt()
           Present.build_documents(doc_session._2, deps, store, progress = progress)
           None
         }
         catch {
           case Exn.Interrupt.ERROR(msg) =>
             val sep = if (msg.contains('\n')) "\n" else " "
             Some("Documentation " + doc_session._1 + " failed:" + sep + msg)
         }, selected).flatten
 
     if (errs.nonEmpty) error(cat_lines(errs))
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_doc", "build Isabelle documentation", args =>
     {
       var all_docs = false
       var max_jobs = 1
       var options = Options.init()
 
       val getopts =
         Getopts("""
 Usage: isabelle build_doc [OPTIONS] [DOCS ...]
 
   Options are:
     -a           select all documentation sessions
     -j INT       maximum number of parallel jobs (default 1)
     -o OPTION    override Isabelle system OPTION (via NAME=VAL or NAME)
 
   Build Isabelle documentation from documentation sessions with
   suitable document_variants entry.
 """,
           "a" -> (_ => all_docs = true),
           "j:" -> (arg => max_jobs = Value.Int.parse(arg)),
           "o:" -> (arg => options = options + arg))
 
       val docs = getopts(args)
 
       if (!all_docs && docs.isEmpty) getopts.usage()
 
       val progress = new Console_Progress()
 
       progress.interrupt_handler {
         build_doc(options, progress, all_docs, max_jobs, docs)
       }
     })
 }
diff --git a/src/Pure/System/progress.scala b/src/Pure/System/progress.scala
--- a/src/Pure/System/progress.scala
+++ b/src/Pure/System/progress.scala
@@ -1,84 +1,88 @@
 /*  Title:      Pure/System/progress.scala
     Author:     Makarius
 
 Progress context for system processes.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 
 
 object Progress
 {
   sealed case class Theory(theory: String, session: String = "", percentage: Option[Int] = None)
   {
     def message: String = print_session + print_theory + print_percentage
 
     def print_session: String = if (session == "") "" else session + ": "
     def print_theory: String = "theory " + theory
     def print_percentage: String =
       percentage match { case None => "" case Some(p) => " " + p + "%" }
   }
 }
 
 class Progress
 {
   def echo(msg: String) {}
   def echo_if(cond: Boolean, msg: String) { if (cond) echo(msg) }
   def theory(theory: Progress.Theory) {}
   def nodes_status(nodes_status: Document_Status.Nodes_Status) {}
 
   def echo_warning(msg: String) { echo(Output.warning_text(msg)) }
   def echo_error_message(msg: String) { echo(Output.error_message_text(msg)) }
   def echo_document(path: Path) { echo("Document at " + path.absolute) }
 
   def timeit[A](message: String = "", enabled: Boolean = true)(e: => A): A =
     Timing.timeit(message, enabled, echo)(e)
 
   @volatile protected var is_stopped = false
   def stop { is_stopped = true }
   def stopped: Boolean =
   {
     if (Thread.interrupted) is_stopped = true
     is_stopped
   }
 
   def interrupt_handler[A](e: => A): A = POSIX_Interrupt.handler { stop } { e }
   def expose_interrupt() { if (stopped) throw Exn.Interrupt() }
   override def toString: String = if (stopped) "Progress(stopped)" else "Progress"
 
   def bash(script: String,
     cwd: JFile = null,
     env: Map[String, String] = Isabelle_System.settings(),
     redirect: Boolean = false,
     echo: Boolean = false,
+    watchdog: Time = Time.zero,
     strict: Boolean = true): Process_Result =
   {
-    Isabelle_System.bash(script, cwd = cwd, env = env, redirect = redirect,
-      progress_stdout = echo_if(echo, _),
-      progress_stderr = echo_if(echo, _),
-      strict = strict)
+    val result =
+      Isabelle_System.bash(script, cwd = cwd, env = env, redirect = redirect,
+        progress_stdout = echo_if(echo, _),
+        progress_stderr = echo_if(echo, _),
+        watchdog = if (watchdog.is_zero) None else Some((watchdog, _ => stopped)),
+        strict = strict)
+    if (strict && stopped) throw Exn.Interrupt() else result
   }
 }
 
 class Console_Progress(verbose: Boolean = false, stderr: Boolean = false) extends Progress
 {
   override def echo(msg: String): Unit =
     Output.writeln(msg, stdout = !stderr, include_empty = true)
 
   override def theory(theory: Progress.Theory): Unit =
     if (verbose) echo(theory.message)
 }
 
 class File_Progress(path: Path, verbose: Boolean = false) extends Progress
 {
   override def echo(msg: String): Unit =
     File.append(path, msg + "\n")
 
   override def theory(theory: Progress.Theory): Unit =
     if (verbose) echo(theory.message)
 
   override def toString: String = path.toString
 }
diff --git a/src/Pure/Thy/present.scala b/src/Pure/Thy/present.scala
--- a/src/Pure/Thy/present.scala
+++ b/src/Pure/Thy/present.scala
@@ -1,396 +1,397 @@
 /*  Title:      Pure/Thy/present.scala
     Author:     Makarius
 
 Theory presentation: HTML.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile}
 
 import scala.collection.immutable.SortedMap
 
 
 object Present
 {
   /* maintain chapter index -- NOT thread-safe */
 
   private val sessions_path = Path.basic(".sessions")
 
   private def read_sessions(dir: Path): List[(String, String)] =
   {
     val path = dir + sessions_path
     if (path.is_file) {
       import XML.Decode._
       list(pair(string, string))(Symbol.decode_yxml(File.read(path)))
     }
     else Nil
   }
 
   private def write_sessions(dir: Path, sessions: List[(String, String)])
   {
     import XML.Encode._
     File.write(dir + sessions_path, YXML.string_of_body(list(pair(string, string))(sessions)))
   }
 
   def update_chapter_index(browser_info: Path, chapter: String, new_sessions: List[(String, String)])
   {
     val dir = Isabelle_System.make_directory(browser_info + Path.basic(chapter))
 
     val sessions0 =
       try { read_sessions(dir) }
       catch { case _: XML.Error => Nil }
 
     val sessions = (SortedMap.empty[String, String] ++ sessions0 ++ new_sessions).toList
     write_sessions(dir, sessions)
 
     val title = "Isabelle/" + chapter + " sessions"
     HTML.write_document(dir, "index.html",
       List(HTML.title(title + " (" + Distribution.version + ")")),
       HTML.chapter(title) ::
        (if (sessions.isEmpty) Nil
         else
           List(HTML.div("sessions",
             List(HTML.description(
               sessions.map({ case (name, description) =>
                 val descr = Symbol.trim_blank_lines(description)
                 (List(HTML.link(name + "/index.html", HTML.text(name))),
                   if (descr == "") Nil
                   else HTML.break ::: List(HTML.pre(HTML.text(descr)))) })))))))
   }
 
   def make_global_index(browser_info: Path)
   {
     if (!(browser_info + Path.explode("index.html")).is_file) {
       Isabelle_System.make_directory(browser_info)
       File.copy(Path.explode("~~/lib/logo/isabelle.gif"),
         browser_info + Path.explode("isabelle.gif"))
       File.write(browser_info + Path.explode("index.html"),
         File.read(Path.explode("~~/lib/html/library_index_header.template")) +
         File.read(Path.explode("~~/lib/html/library_index_content.template")) +
         File.read(Path.explode("~~/lib/html/library_index_footer.template")))
     }
   }
 
 
   /* finish session */
 
   def finish(
     browser_info: Path,
     graph_pdf: Bytes,
     info: Sessions.Info,
     name: String)
   {
     val session_prefix = browser_info + Path.basic(info.chapter) + Path.basic(name)
     val session_fonts = session_prefix + Path.explode("fonts")
 
     if (info.options.bool("browser_info")) {
       Isabelle_System.make_directory(session_fonts)
 
       Bytes.write(session_prefix + session_graph_path, graph_pdf)
 
       HTML.write_isabelle_css(session_prefix)
 
       for (entry <- Isabelle_Fonts.fonts(hidden = true))
         File.copy(entry.path, session_fonts)
     }
   }
 
 
   /** preview **/
 
   sealed case class Preview(title: String, content: String)
 
   def preview(
     resources: Resources,
     snapshot: Document.Snapshot,
     plain_text: Boolean = false,
     fonts_url: String => String = HTML.fonts_url()): Preview =
   {
     require(!snapshot.is_outdated)
 
     def output_document(title: String, body: XML.Body): String =
       HTML.output_document(
         List(
           HTML.style(HTML.fonts_css(fonts_url) + "\n\n" + File.read(HTML.isabelle_css)),
           HTML.title(title)),
         List(HTML.source(body)), css = "", structural = false)
 
     val name = snapshot.node_name
 
     if (plain_text) {
       val title = "File " + quote(name.path.file_name)
       val content = output_document(title, HTML.text(snapshot.node.source))
       Preview(title, content)
     }
     else {
       resources.make_preview(snapshot) match {
         case Some(preview) => preview
         case None =>
           val title =
             if (name.is_theory) "Theory " + quote(name.theory_base_name)
             else "File " + quote(name.path.file_name)
           val content = output_document(title, pide_document(snapshot))
           Preview(title, content)
       }
     }
   }
 
 
   /* PIDE document */
 
   private val document_elements =
     Rendering.foreground_elements ++ Rendering.text_color_elements ++ Rendering.markdown_elements +
     Markup.NUMERAL + Markup.COMMENT + Markup.LANGUAGE
 
   private val div_elements =
     Set(HTML.div.name, HTML.pre.name, HTML.par.name, HTML.list.name, HTML.enum.name,
       HTML.descr.name)
 
   private def html_div(html: XML.Body): Boolean =
     html exists {
       case XML.Elem(markup, body) => div_elements.contains(markup.name) || html_div(body)
       case XML.Text(_) => false
     }
 
   private def html_class(c: String, html: XML.Body): XML.Tree =
     if (html.forall(_ == HTML.no_text)) HTML.no_text
     else if (html_div(html)) HTML.div(c, html)
     else HTML.span(c, html)
 
   private def make_html(xml: XML.Body): XML.Body =
     xml map {
       case XML.Elem(Markup(Markup.LANGUAGE, Markup.Name(Markup.Language.DOCUMENT)), body) =>
         html_class(Markup.Language.DOCUMENT, make_html(body))
       case XML.Elem(Markup(Markup.MARKDOWN_PARAGRAPH, _), body) => HTML.par(make_html(body))
       case XML.Elem(Markup(Markup.MARKDOWN_ITEM, _), body) => HTML.item(make_html(body))
       case XML.Elem(Markup(Markup.Markdown_Bullet.name, _), _) => HTML.no_text
       case XML.Elem(Markup.Markdown_List(kind), body) =>
         if (kind == Markup.ENUMERATE) HTML.enum(make_html(body)) else HTML.list(make_html(body))
       case XML.Elem(markup, body) =>
         val name = markup.name
         val html =
           markup.properties match {
             case Markup.Kind(kind) if kind == Markup.COMMAND || kind == Markup.KEYWORD =>
               List(html_class(kind, make_html(body)))
             case _ =>
               make_html(body)
           }
         Rendering.foreground.get(name) orElse Rendering.text_color.get(name) match {
           case Some(c) => html_class(c.toString, html)
           case None => html_class(name, html)
         }
       case XML.Text(text) =>
         XML.Text(Symbol.decode(text))
     }
 
   def pide_document(snapshot: Document.Snapshot): XML.Body =
     make_html(snapshot.markup_to_XML(Text.Range.full, document_elements))
 
 
 
   /** build documents **/
 
   val session_tex_path = Path.explode("session.tex")
   val session_graph_path = Path.explode("session_graph.pdf")
 
   def tex_name(name: Document.Node.Name): String = name.theory_base_name + ".tex"
   def document_tex_name(name: Document.Node.Name): String = "document/" + tex_name(name)
 
   def isabelletags(tags: List[String]): String =
     Library.terminate_lines(
       tags.map(tag =>
         tag.toList match {
           case '/' :: cs => "\\isafoldtag{" + cs.mkString + "}"
           case '-' :: cs => "\\isadroptag{" + cs.mkString + "}"
           case '+' :: cs => "\\isakeeptag{" + cs.mkString + "}"
           case cs => "\\isakeeptag{" + cs.mkString + "}"
         }))
 
   def build_documents(
     session: String,
     deps: Sessions.Deps,
     store: Sessions.Store,
     progress: Progress = new Progress,
     verbose: Boolean = false,
     verbose_latex: Boolean = false): List[(String, Bytes)] =
   {
     /* session info */
 
     val info = deps.sessions_structure(session)
     val options = info.options
     val base = deps(session)
     val graph_pdf = graphview.Graph_File.make_pdf(options, base.session_graph_display)
 
 
     /* prepare document directory */
 
     def prepare_dir(dir: Path, doc_name: String, doc_tags: List[String]): (Path, String) =
     {
       val doc_dir = dir + Path.basic(doc_name)
       Isabelle_System.make_directory(doc_dir)
 
       Isabelle_System.bash("isabelle latex -o sty", cwd = doc_dir.file).check
       File.write(doc_dir + Path.explode("isabelletags.sty"), isabelletags(doc_tags))
       for ((base_dir, src) <- info.document_files) File.copy_base(info.dir + base_dir, src, doc_dir)
       Bytes.write(doc_dir + session_graph_path, graph_pdf)
 
       File.write(doc_dir + session_tex_path,
         Library.terminate_lines(
           base.session_theories.map(name => "\\input{" + tex_name(name) + "}")))
 
       using(store.open_database(session))(db =>
         for (name <- base.session_theories) {
           val tex =
             Export.read_entry(db, session, name.theory, document_tex_name(name)) match {
               case Some(entry) => entry.uncompressed(cache = store.xz_cache)
               case None => Bytes.empty
             }
           Bytes.write(doc_dir + Path.basic(tex_name(name)), tex)
         })
 
       val root1 = "root_" + doc_name
       val root = if ((doc_dir + Path.explode(root1).tex).is_file) root1 else "root"
 
       (doc_dir, root)
     }
 
 
     /* produce documents */
 
     val doc_output = info.document_output
 
     val docs =
       for ((doc_name, doc_tags) <- info.documents)
       yield {
         Isabelle_System.with_tmp_dir("document")(tmp_dir =>
         {
           val (doc_dir, root) = prepare_dir(tmp_dir, doc_name, doc_tags)
           doc_output.foreach(prepare_dir(_, doc_name, doc_tags))
 
 
           // bash scripts
 
           def root_bash(ext: String): String = Bash.string(root + "." + ext)
 
           def latex_bash(fmt: String = "pdf", ext: String = "tex"): String =
             "isabelle latex -o " + Bash.string(fmt) + " " + Bash.string(root + "." + ext)
 
           def bash(items: String*): Process_Result =
-            progress.bash(items.mkString(" && "), cwd = doc_dir.file, echo = verbose_latex)
+            progress.bash(items.mkString(" && "), cwd = doc_dir.file,
+              echo = verbose_latex, watchdog = Time.seconds(0.5))
 
 
           // prepare document
 
           val result =
             if ((doc_dir + Path.explode("build")).is_file) {
               bash("./build pdf " + Bash.string(doc_name))
             }
             else {
               bash(
                 latex_bash(),
                 "{ [ ! -f " + root_bash("bib") + " ] || " + latex_bash("bbl") + "; }",
                 "{ [ ! -f " + root_bash("idx") + " ] || " + latex_bash("idx") + "; }",
                 latex_bash(),
                 latex_bash())
             }
 
 
           // result
 
           val root_pdf = Path.basic(root).pdf
           val result_path = doc_dir + root_pdf
 
           if (!result.ok) {
             cat_error(
               Library.trim_line(result.err),
               cat_lines(Latex.latex_errors(doc_dir, root) ::: Bibtex.bibtex_errors(doc_dir, root)),
               "Failed to build document " + quote(doc_name))
           }
           else if (!result_path.is_file) {
             error("Bad document result: expected to find " + root_pdf)
           }
           else doc_name -> Bytes.read(result_path)
         })
       }
 
     def output(dir: Path)
     {
       Isabelle_System.make_directory(dir)
       for ((name, pdf) <- docs) {
         val path = dir + Path.basic(name).pdf
         Bytes.write(path, pdf)
         progress.echo_document(path)
       }
     }
 
     if (info.options.bool("browser_info") || doc_output.isEmpty) {
       output(store.browser_info + Path.basic(info.chapter) + Path.basic(session))
     }
 
     doc_output.foreach(output)
 
     docs
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("document", "prepare session theory document", args =>
     {
       var verbose_latex = false
       var dirs: List[Path] = Nil
       var no_build = false
       var options = Options.init()
       var verbose_build = false
 
       val getopts = Getopts(
         """
 Usage: isabelle document [OPTIONS] SESSION
 
   Options are:
     -O           set option "document_output", relative to current directory
     -V           verbose latex
     -d DIR       include session directory
     -n           no build of session
     -o OPTION    override Isabelle system OPTION (via NAME=VAL or NAME)
     -v           verbose build
 
   Prepare the theory document of a session.
 """,
         "O:" -> (arg => options += ("document_output=" + Path.explode(arg).absolute.implode)),
         "V" -> (_ => verbose_latex = true),
         "d:" -> (arg => dirs = dirs ::: List(Path.explode(arg))),
         "n" -> (_ => no_build = true),
         "o:" -> (arg => options = options + arg),
         "v" -> (_ => verbose_build = true))
 
       val more_args = getopts(args)
       val session =
         more_args match {
           case List(a) => a
           case _ => getopts.usage()
         }
 
       val progress = new Console_Progress(verbose = verbose_build)
       val store = Sessions.store(options)
 
       progress.interrupt_handler {
         if (!no_build) {
           val res =
             Build.build(options, selection = Sessions.Selection.session(session),
               dirs = dirs, progress = progress, verbose = verbose_build)
           if (!res.ok) error("Failed to build session " + quote(session))
         }
 
         val deps =
           Sessions.load_structure(options + "document=pdf", dirs = dirs).
             selection_deps(Sessions.Selection.session(session))
 
         build_documents(session, deps, store, progress = progress,
           verbose = true, verbose_latex = verbose_latex)
       }
     })
 }