src/Pure/GUI/gui.scala

--- a/src/Pure/GUI/gui.scala	Mon Nov 03 17:17:53 2025 +0100
+++ b/src/Pure/GUI/gui.scala	Mon Nov 03 18:03:05 2025 +0100
@@ -385,7 +385,7 @@
 
   def tooltip_lines(text: String): String =
     if (text == null || text == "") null
-    else Style_HTML.enclose_text(text)
+    else Style_HTML.enclose(split_lines(text).map(Style_HTML.make_text).mkString("<br/>"))
 
 
   /* icon */