Tweak doc watch mode handling

build.sc

@@ -511,9 +511,19 @@ object docs extends ScalaModule {
   }
   def mkdocsServe() = T.command[Unit] {
     mdoc()()
-    os.proc("mkdocs", "serve", mkdocsConfigArgs())
-      .call(cwd = millModuleBasePath.value, stdin = os.Inherit, stdout = os.Inherit)
-
+    val docsDir = T.workspace / "docs"
+    val serveProc = os.proc("mkdocs", "serve", mkdocsConfigArgs())
+      .spawn(cwd = docsDir, stdin = os.Inherit, stdout = os.Inherit)
+    val mdocProc = os.proc("java", "-cp", compileClasspath().map(_.path).mkString(File.pathSeparator), mainClass().getOrElse(???), mdocWatchArgs().value)
+      .spawn(cwd = docsDir, stdin = os.Inherit, stdout = os.Inherit)
+
+    while (serveProc.isAlive && mdocProc.isAlive)
+      Thread.sleep(1000L)
+
+    val serveRetCode = serveProc.waitFor()
+    val mdocRetCode = mdocProc.waitFor()
+    if (serveRetCode != 0 || mdocRetCode != 0)
+      sys.error(s"Got exit code $serveRetCode for mkdocs serve and $mdocRetCode for mdoc")
     ()
   }
   def mkdocsBuild() = T.command[Unit] {

docs/pages/develop.md

@@ -4,14 +4,9 @@
 
 ### Watch mode
 
-In a first terminal, run
+Run
 ```text
-$ ./mill -i -w docs.mdocWatch
-```
-
-Leave it running, and run in a second terminal
-```text
-$ ./mill -i -w docs.mkdocsServe
+$ ./mill -i docs.mkdocsServe
 ```
 
 Then open the URL printed in the console in your browser (it should be