[2.x] fix: Fixes plugin toggle precedence between disablePlugins and enablePlugins (#8794)

Treat explicit plugin toggles as last-call-wins for the same plugin.
This avoids contradictory include/exclude states when disablePlugins(X) is followed by
enablePlugins(X) (and vice versa), aligning behavior with normal override expectations.

Apply the same semantics to ProjectMatrix and add regression coverage:
- unit tests in main/src/test/scala/ProjectSpec.scala
- scripted test in sbt-app/src/sbt-test/project/i1926-disable-enable-plugin

main-settings/src/main/scala/sbt/Plugins.scala

@@ -343,6 +343,16 @@ ${listConflicts(conflicting)}""")
     case And(ns)  => ns.foldLeft(a)(_ && _)
     case b: Basic => a && b
   }
+
+  private[sbt] def overrideWith(current: Plugins, update: Plugins): Plugins = {
+    val opposite: Set[Basic] = flatten(update).map {
+      case Exclude(p) => p: Basic
+      case p: AutoPlugin =>
+        Exclude(p): Basic
+    }.toSet
+    and(remove(current, opposite), update)
+  }
+
   private[sbt] def remove(a: Plugins, del: Set[Basic]): Plugins = a match {
     case b: Basic => if (del(b)) Empty else b
     case Empty    => Empty

main-settings/src/main/scala/sbt/Project.scala

@@ -158,11 +158,18 @@ sealed trait Project extends ProjectDefinition[ProjectReference] with CompositeP
    * A [[AutoPlugin]] is a common label that is used by plugins to determine what settings, if any, to enable on a project.
    */
   def enablePlugins(ns: Plugins*): Project =
-    setPlugins(ns.foldLeft(plugins)(Plugins.and))
+    setPlugins(ns.foldLeft(plugins)(Plugins.overrideWith))
 
   /** Disable the given plugins on this project. */
   def disablePlugins(ps: AutoPlugin*): Project =
-    setPlugins(Plugins.and(plugins, Plugins.And(ps.map(p => Plugins.Exclude(p)).toList)))
+    if ps.isEmpty then this
+    else
+      setPlugins(
+        Plugins.overrideWith(
+          plugins,
+          Plugins.And(ps.map(p => Plugins.Exclude(p)).toList)
+        )
+      )
 
   private[sbt] def setPlugins(ns: Plugins): Project = copy(plugins = ns)
 

main/src/main/scala/sbt/ProjectMatrix.scala

@@ -455,10 +455,17 @@ object ProjectMatrix {
       copy(settings = (settings: Seq[Def.Setting[?]]) ++ Def.settings(ss*))
 
     override def enablePlugins(ns: Plugins*): ProjectMatrix =
-      setPlugins(ns.foldLeft(plugins)(Plugins.and))
+      setPlugins(ns.foldLeft(plugins)(Plugins.overrideWith))
 
     override def disablePlugins(ps: AutoPlugin*): ProjectMatrix =
-      setPlugins(Plugins.and(plugins, Plugins.And(ps.map(p => Plugins.Exclude(p)).toList)))
+      if ps.isEmpty then this
+      else
+        setPlugins(
+          Plugins.overrideWith(
+            plugins,
+            Plugins.And(ps.map(p => Plugins.Exclude(p)).toList)
+          )
+        )
 
     override def configure(ts: (Project => Project)*): ProjectMatrix =
       copy(transforms = transforms ++ ts)

main/src/test/scala/ProjectSpec.scala

@@ -8,10 +8,30 @@
 
 package sbt
 
+import java.io.File
+
 object ProjectSpec extends verify.BasicTestSuite {
+  object TestPlugin extends AutoPlugin {
+    override def requires: Plugins = empty
+  }
+
+  private val base = new File(".")
+
   test("Project should normalize projectIDs if they are empty") {
     assert(Project.normalizeProjectID(emptyFilename) == Right("root"))
   }
 
+  test("disablePlugins then enablePlugins should keep plugin enabled") {
+    val p = Project("test", base).disablePlugins(TestPlugin).enablePlugins(TestPlugin)
+    assert(Plugins.hasInclude(p.plugins, TestPlugin))
+    assert(!Plugins.hasExclude(p.plugins, TestPlugin))
+  }
+
+  test("enablePlugins then disablePlugins should keep plugin disabled") {
+    val p = Project("test", base).enablePlugins(TestPlugin).disablePlugins(TestPlugin)
+    assert(!Plugins.hasInclude(p.plugins, TestPlugin))
+    assert(Plugins.hasExclude(p.plugins, TestPlugin))
+  }
+
   def emptyFilename = ""
 }

sbt-app/src/sbt-test/project/i1926-disable-enable-plugin/broken.sbt.disabled

@@ -0,0 +1,4 @@
+def myProject(id: String): Project =
+  Project(id, file(id)).disablePlugins(Issue1926Plugin)
+
+lazy val b = myProject("b").enablePlugins(Issue1926Plugin)

sbt-app/src/sbt-test/project/i1926-disable-enable-plugin/build.sbt

@@ -0,0 +1,4 @@
+def myProject(id: String): Project =
+  Project(id, file(id)).disablePlugins(Issue1926Plugin)
+
+lazy val a = myProject("a")

sbt-app/src/sbt-test/project/i1926-disable-enable-plugin/project/Issue1926Plugin.scala

@@ -0,0 +1,13 @@
+import sbt._
+
+object Issue1926Plugin extends AutoPlugin {
+  object autoImport {
+    val issue1926Marker = settingKey[String]("Marker setting for issue #1926 test")
+  }
+
+  import autoImport._
+
+  override def requires: Plugins = plugins.JvmPlugin
+  override def trigger = noTrigger
+  override def projectSettings = Seq(issue1926Marker := "enabled")
+}

sbt-app/src/sbt-test/project/i1926-disable-enable-plugin/test

@@ -0,0 +1,4 @@
+> projects
+$ copy-file broken.sbt.disabled broken.sbt
+> reload
+> show b/issue1926Marker