[2.x] Add scripted test for #7838 (#8541)

This PR adds a new scripted test `watch-termination-reload` to prevent regression of the fix in PR #7838. The test verifies that the `Reload` action is correctly passed to the `watchOnTermination` callback when a reload is triggered in watch mode, instead of falling back to `CancelWatch` (which was the root cause of issue #7017).

src/sbt-test/watch/watch-termination-reload/project/Build.scala

@@ -0,0 +1,43 @@
+package sbt.watch.termination.reload
+
+import sbt._
+import Keys._
+import sbt.nio.Keys._
+import sbt.nio.Watch
+
+object Build {
+  val checkTerminationAction = inputKey[Unit]("Check that watchOnTermination received the expected action")
+  val checkTerminationActionImpl: Def.Initialize[InputTask[Unit]] = Def.inputTask {
+    val Seq(expectedAction) = Def.spaceDelimited().parsed
+    val logFile = baseDirectory.value / "action-log.txt"
+    val loggedAction = IO.read(logFile).trim
+    assert(loggedAction == expectedAction, s"Expected action '$expectedAction', but got '$loggedAction'")
+  }
+
+  lazy val root = (project in file(".")).settings(
+    checkTerminationAction := checkTerminationActionImpl.evaluated,
+    watchOnTermination := { (count: Int, action: Watch.Action) =>
+      if (action == Watch.Reload) {
+        val logFile = baseDirectory.value / "action-log.txt"
+        IO.write(logFile, action.toString)
+      }
+    },
+    watchOnFileInputEvent := { (count: Int, event: Watch.Event) =>
+      if (event.path.getFileName.toString == "trigger.txt") {
+        val flagFile = baseDirectory.value / "triggered.txt"
+        if (!flagFile.exists) {
+          IO.write(flagFile, "true")
+          Watch.Reload
+        } else {
+          Watch.CancelWatch
+        }
+      } else Watch.Ignore
+    },
+    watchSources += baseDirectory.value / "trigger.txt",
+    Compile / compile := {
+      val trigger = baseDirectory.value / "trigger.txt"
+      IO.write(trigger, System.currentTimeMillis.toString)
+      (Compile / compile).value
+    }
+  )
+}

src/sbt-test/watch/watch-termination-reload/src/main/scala/Test.scala

@@ -0,0 +1,3 @@
+class Test {
+  // Minimal source to enable ~compile in watch mode
+}

src/sbt-test/watch/watch-termination-reload/test

@@ -0,0 +1,4 @@
+# Start watch mode with ~compile, which triggers file modification and reload
+> ~compile
+# Check that watchOnTermination received Reload action
+> checkTerminationAction Reload