Steps to reproduce

1. run zssapp in zssee-app-eval-3.8.0-FL-2015-03-09
2. user 1 open the file, mortgage_refinancing.xlsx
3. user 2 open the same file
4. user 2 add a sheet
5. user 2 keeps typing characters in many cells, like the screenshot below
   I type over 300 characters to reproduce this issue and type quickly with the number key and arrow key.
   

Actual Result

when you keep typing, at a certain moment, zssapp shows "Processing..." and never disappear. All input are blocked, and you cannot enter any character in a cell.

Debug Information

chrome console doesn't show any javascript error, no Java exception found on an application console.
When the issue happens, one comet request and one ZK au is pending and no response received.
Client side profiling reveals that it's idle.

The AU details:

According to threadDump.txt, all ZK related threads are in WAITING state

"http-bio-8080-exec-9" daemon prio=6 tid=0x0000000007d9a000 nid=0x41d0 in Object.wait() [0x000000000b59e000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)

• waiting on <0x0000000086d78070> (a java.lang.Object)
  at java.lang.Object.wait(Object.java:485)
  at org.zkoss.zkex.ui.comet.CometServerPush.processRequest(CometServerPush.java:359)
• locked <0x0000000086d78070> (a java.lang.Object)
  ...

"http-bio-8080-exec-7" daemon prio=6 tid=0x0000000007d98800 nid=0x3570 in Object.wait() [0x000000000b39e000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)

• waiting on <0x0000000086c63958> (a java.util.LinkedList)
  at java.lang.Object.wait(Object.java:485)
  at org.zkoss.zkex.ui.comet.CometServerPush.processRequest(CometServerPush.java:319)
• locked <0x0000000086c63958> (a java.util.LinkedList)
  at org.zkoss.zkex.ui.comet.CometExtension.service0(CometExtension.java:135)
  ...

"pool-1-thread-1" prio=6 tid=0x0000000007d96800 nid=0x2600 in Object.wait() [0x000000000b09e000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)

• waiting on <0x0000000086d78350> (a java.lang.Object)
  at org.zkoss.zk.ui.impl.UiEngineImpl.doActivate(UiEngineImpl.java:1872)
• locked <0x0000000086d78350> (a java.lang.Object)
  at org.zkoss.zk.ui.impl.UiEngineImpl.beginUpdate(UiEngineImpl.java:1144)
  at org.zkoss.zkex.ui.comet.CometServerPush.activate(CometServerPush.java:447)

"http-bio-8080-exec-16" daemon prio=6 tid=0x0000000007d94000 nid=0x2d4c waiting on condition [0x0000000008fde000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)

• parking to wait for <0x0000000086c1a2d8> (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:811)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:842)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1178)
  at java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock.lock(ReentrantReadWriteLock.java:807)
  at org.zkoss.zss.model.util.ReadWriteTask.doInWriteLock(ReadWriteTask.java:41)
  at org.zkoss.zss.range.impl.RangeImpl.notifyCustomEvent(RangeImpl.java:1595)
  at org.zkoss.zss.ui.Spreadsheet.moveSelfEditorFocus(Spreadsheet.java:694)
  at org.zkoss.zss.ui.Spreadsheet.doMoveSelfFocus(Spreadsheet.java:704)
  at org.zkoss.zss.ui.Spreadsheet.access$600(Spreadsheet.java:223)
  at org.zkoss.zss.ui.Spreadsheet$5.onEvent(Spreadsheet.java:785)
  ...

"http-bio-8080-exec-2" daemon prio=6 tid=0x0000000007d92000 nid=0x40fc waiting on condition [0x000000000ab9e000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)

• parking to wait for <0x0000000086c1a2d8> (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:811)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:842)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1178)
  at java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock.lock(ReentrantReadWriteLock.java:807)
  at org.zkoss.zss.model.util.ReadWriteTask.doInWriteLock(ReadWriteTask.java:41)
  at org.zkoss.zss.range.impl.RangeImpl.setEditText(RangeImpl.java:437)
  at org.zkoss.zss.api.impl.RangeImpl.setCellEditText(RangeImpl.java:538)
  at org.zkoss.zss.ui.impl.undo.CellEditTextAction.applyAction(CellEditTextAction.java:52)
  at org.zkoss.zss.ui.impl.undo.AbstractEditTextAction.doAction(AbstractEditTextAction.java:93)
  at org.zkoss.zssex.ui.impl.UndoableActionManagerImpl.doAction(UndoableActionManagerImpl.java:50)
  at org.zkoss.zss.ui.Spreadsheet.processStopEditing0(Spreadsheet.java:4755)