diff --git a/util/io/src/main/scala/sbt/SourceModificationWatch.scala b/util/io/src/main/scala/sbt/SourceModificationWatch.scala
index f26562294..e24452f63 100644
--- a/util/io/src/main/scala/sbt/SourceModificationWatch.scala
+++ b/util/io/src/main/scala/sbt/SourceModificationWatch.scala
@@ -10,13 +10,14 @@ object SourceModificationWatch {
     {
       import state._
 
-      def sourceFiles: Iterable[java.io.File] = sourcesFinder.get
-      val (lastModifiedTime, fileCount) =
-        ((0L, 0) /: sourceFiles) { (acc, file) => (math.max(acc._1, file.lastModified), acc._2 + 1) }
+      val sourceFiles: Iterable[java.io.File] = sourcesFinder.get
+      val sourceFilesPath: Set[String] = sourceFiles.map(_.getCanonicalPath)(collection.breakOut)
+      val lastModifiedTime =
+        (0L /: sourceFiles) { (acc, file) => math.max(acc, file.lastModified) }
 
       val sourcesModified =
         lastModifiedTime > lastCallbackCallTime ||
-          previousFileCount != fileCount
+          previousFiles != sourceFilesPath
 
       val (triggered, newCallbackCallTime) =
         if (sourcesModified)
@@ -24,7 +25,7 @@ object SourceModificationWatch {
         else
           (awaitingQuietPeriod, lastCallbackCallTime)
 
-      val newState = new WatchState(newCallbackCallTime, fileCount, sourcesModified, if (triggered) count + 1 else count)
+      val newState = new WatchState(newCallbackCallTime, sourceFilesPath, sourcesModified, if (triggered) count + 1 else count)
       if (triggered)
         (true, newState)
       else {
@@ -36,7 +37,16 @@ object SourceModificationWatch {
       }
     }
 }
-final class WatchState(val lastCallbackCallTime: Long, val previousFileCount: Int, val awaitingQuietPeriod: Boolean, val count: Int)
+final class WatchState(val lastCallbackCallTime: Long, val previousFiles: Set[String], val awaitingQuietPeriod: Boolean, val count: Int) {
+
+  def previousFileCount: Int = previousFiles.size
+
+  @deprecated("Use another constructor", "0.13.6")
+  def this(lastCallbackCallTime: Long, previousFileCount: Int, awaitingQuietPeriod: Boolean, count: Int) {
+    this(lastCallbackCallTime, Set.empty[String], awaitingQuietPeriod, count)
+  }
+}
+
 object WatchState {
-  def empty = new WatchState(0L, 0, false, 0)
-}
\ No newline at end of file
+  def empty = new WatchState(0L, Set.empty[String], false, 0)
+}