diff -r 908351c8c0b1 -r d83e7e444b43 src/Pure/Thy/sessions.scala
--- a/src/Pure/Thy/sessions.scala	Sun May 16 13:06:13 2021 +0200
+++ b/src/Pure/Thy/sessions.scala	Sun May 16 13:14:16 2021 +0200
@@ -502,7 +502,9 @@
 
     def dirs: List[Path] = dir :: directories
 
-    def timeout_ignored: Boolean = Time.seconds(options.real("timeout")) < Time.ms(1)
+    def timeout_ignored: Boolean =
+      !options.bool("timeout_build") || Time.seconds(options.real("timeout")) < Time.ms(1)
+
     def timeout: Time = Time.seconds(options.real("timeout") * options.real("timeout_scale"))
 
     def document_enabled: Boolean =