diff --git a/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java b/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java
index 7d9450983f..20fe9e1c98 100644
--- a/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java
+++ b/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java
@@ -111,7 +111,7 @@ public void dump(OutputStream os) throws IOException {
         }
         end.centot = elist.size();
         end.cenlen = written;
-        end.write(os, written);
+        end.write(os, written + end.cenoff);
     }
 
     private List<Entry> readEntries() throws IOException {
@@ -242,6 +242,7 @@ private END findEND() throws IOException
                         end.centot = (int)ZIP64_ENDTOT(end64buf); // assume total < 2g
                         end.endpos = end64pos;
                     }
+                    log(end);
                     return end;
                 }
             }
@@ -431,6 +432,8 @@ void write(OutputStream os, long offset) throws IOException {
                 writeShort(os, 0);
             }
         }
+
+        public String toString() { return String.format("END[@%07d #%05d %07dB]", cenoff, centot, cenlen); }
     }
 
     public static class Entry extends IndexNode {
@@ -739,4 +742,7 @@ void readExtra(ZipCentralDir zipfs) throws IOException {
         }
     }
 
+    private static void log(Object x) {
+        //System.out.println(x);
+    }
 }
diff --git a/internal/zinc-classfile/src/test/scala/sbt/internal/inc/classfile/IndexBasedZipFsOpsSpec.scala b/internal/zinc-classfile/src/test/scala/sbt/internal/inc/classfile/IndexBasedZipFsOpsSpec.scala
new file mode 100644
index 0000000000..3402d18715
--- /dev/null
+++ b/internal/zinc-classfile/src/test/scala/sbt/internal/inc/classfile/IndexBasedZipFsOpsSpec.scala
@@ -0,0 +1,75 @@
+package sbt.internal.inc
+package classfile
+
+import java.nio.file._
+import java.nio.file.spi.FileSystemProvider
+import scala.collection.JavaConverters._
+
+class IndexBasedZipFsOpsSpec extends UnitSpec {
+  private val XL = 0xffff // minimum size to be zip64, which I'm calling "XL"
+  private val L = XL - 1 // last size to be standard zip, which I'm calling "L"
+  private val tmpDir = Files.createTempDirectory("zinc-zipmergetest")
+  private lazy val zipFsProvider =
+    FileSystemProvider.installedProviders().stream().filter(_.getScheme == "jar").findAny().get()
+
+  it should "create XS jars" in assertSize(createJar(2), 2)
+  it should "merge  XS jars" in assertMerge(2, 1)
+  it should "shrink XS jars" in assertShrink(3, 2)
+
+  it should "create  L jars" in assertSize(createJar(L), L)
+  it should "merge   L jars" in assertMerge(L, 1) //      breach threshold
+  it should "shrink  L jars" in assertShrink(L + 1, L) // breach threshold back
+
+  it should "create XL jars" in assertSize(createJar(XL), XL)
+  it should "merge  XL jars" in assertMerge(XL, 1)
+  it should "shrink XL jars" in assertShrink(XL + 1, XL)
+
+  private def assertMerge(size1: Int, size2: Int) = {
+    val a = createJar(size1)
+    val b = createJar(size2, "b", size1)
+    safely(IndexBasedZipFsOps.mergeArchives(a, b))
+    assertSize(a, size1 + size2)
+  }
+
+  private def assertShrink(size1: Int, size2: Int) = {
+    val a = createJar(size1)
+    val files = for (i <- size2 until size1) yield classFileName(i)
+    safely(IndexBasedZipFsOps.removeEntries(a.toFile, files))
+    assertSize(a, size2)
+  }
+
+  private def assertSize(p: Path, size: Int) = {
+    val cen = safely(IndexBasedZipFsOps.readCentralDir(p.toFile))
+    assert(cen.getHeaders.size() == size)
+    Files.delete(p)
+  }
+
+  private def createJar(n: Int, name: String = "a", from: Int = 0) = {
+    val out = tmpDir.resolve(s"$name.jar")
+    val zipfs = zipFsProvider.newFileSystem(out, Map("create" -> "true").asJava)
+    val root = zipfs.getRootDirectories.iterator().next()
+    for (i <- from until (from + n)) {
+      val empty = root.resolve(classFileName(i))
+      Files.write(empty, Array.emptyByteArray)
+    }
+    zipfs.close()
+    out
+  }
+
+  private def classFileName(i: Int) = f"C$i%032d.class"
+
+  private def safely[A](op: => A) =
+    try op
+    catch { case ex: java.util.zip.ZipError => throw new ZipException(ex) }
+}
+
+// Avoid java.util.zip.ZipError, which is a VirtualMachineError!!?!
+final class ZipException(val cause: Throwable) extends Exception
+    with scala.util.control.NoStackTrace {
+  override def toString: String = cause.toString
+  override def getCause: Throwable = cause.getCause
+  override def getMessage: String = cause.getMessage
+  override def getStackTrace: Array[StackTraceElement] = cause.getStackTrace
+  override def printStackTrace(s: java.io.PrintStream): Unit = cause.printStackTrace(s)
+  override def printStackTrace(s: java.io.PrintWriter): Unit = cause.printStackTrace(s)
+}