diff --git a/java/org/apache/coyote/http2/Http2UpgradeHandler.java b/java/org/apache/coyote/http2/Http2UpgradeHandler.java
index 4bd58bc289c4..0d7c9e8d087e 100644
--- a/java/org/apache/coyote/http2/Http2UpgradeHandler.java
+++ b/java/org/apache/coyote/http2/Http2UpgradeHandler.java
@@ -172,6 +172,9 @@ class Http2UpgradeHandler extends AbstractStream implements InternalHttpUpgradeH
     private final PingManager pingManager = getPingManager();
     private volatile int newStreamsSinceLastPrune = 0;
     private final Set<Stream> backLogStreams = new HashSet<>();
+    // The sum of the connection allocations requested by the streams in backLogStreams. Any change to a stream's
+    // requested allocation must be mirrored here so releaseBackLog() can correctly determine whether an increment
+    // clears the backlog.
     private long backLogSize = 0;
     // The time at which the connection will timeout unless data arrives before
     // then. -1 means no timeout.
@@ -1332,6 +1335,7 @@ private int allocate(AbstractStream stream, int allocation) {
                 int allocatedThisTime = Math.min(allocation, stream.getConnectionAllocationRequested());
                 stream.setConnectionAllocationRequested(stream.getConnectionAllocationRequested() - allocatedThisTime);
                 stream.setConnectionAllocationMade(stream.getConnectionAllocationMade() + allocatedThisTime);
+                backLogSize -= allocatedThisTime;
                 leftToAllocate = leftToAllocate - allocatedThisTime;
             }
 
diff --git a/test/org/apache/coyote/http2/TestRfc9218.java b/test/org/apache/coyote/http2/TestRfc9218.java
index 9ed9540b2ea0..4734c2ff057e 100644
--- a/test/org/apache/coyote/http2/TestRfc9218.java
+++ b/test/org/apache/coyote/http2/TestRfc9218.java
@@ -187,7 +187,7 @@ public void testPriority() throws Exception {
         trace = trace.replace("21-Body-1365\n", "");
         Assert.assertEquals(0, trace.length());
 
-        // 19 - 5641 body left
+        // 19 - 5461 body left
         // 21 - 4778 body left
 
         // Add 16k to the connection window. Should fully allocate 19 and 21.
@@ -200,6 +200,29 @@ public void testPriority() throws Exception {
             // Dump for debugging purposes
             ioe.printStackTrace();
         }
+
+        output.clearTrace();
+
+        /*
+         * The final window update was 6145 bytes larger than the outstanding backlog. Those bytes must remain available
+         * in the connection window for the next stream.
+         */
+        sendSimpleGetRequest(23);
+        parser.readFrame();
+        // The headers for stream 23 will always arrive. If the surplus was lost, no body will be sent and the read
+        // below would block for the default timeout - so use a short timeout to fail quickly.
+        s.setSoTimeout(1000);
+        parser.readFrame();
+
+        Assert.assertTrue(output.getTrace().contains("23-Body-6145\n"));
+        output.clearTrace();
+
+        // 2047 completes the 8192 byte response body for stream 23, confirming that exactly 6145 surplus bytes were
+        // available in the connection window.
+        sendWindowUpdate(0, 2047);
+        parser.readFrame();
+
+        Assert.assertEquals("23-Body-2047\n23-EndOfStream\n", output.getTrace());
     }