Excel UI freezes after IUIAutomation::AddFocusChangedEventHandler — unsubscribing does not help, only restart fixes it

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Excel UI freezes after IUIAutomation::AddFocusChangedEventHandler — unsubscribing does not help, only restart fixes it

Hello, I've found a reproducible issue in Microsoft Excel 2010–2024.

Problem: After calling IUIAutomation::AddFocusChangedEventHandler from an external process, Excel starts performing formula recalculations on the UI thread. On large workbooks, the Excel interface becomes unresponsive for several seconds to several minutes. Key issue — unsubscribing does NOT fix it: •RemoveFocusChangedEventHandler — has no effect •RemoveAllEventHandlers — has no effect • Force-killing the process that registered the handler — has no effect Excel continues to freeze as if it doesn't clean up its internal subscription state when the handler is removed. The only workaround is restarting Excel after unsubscription. Steps to reproduce: 1. Open a large workbook in Excel 2010–2024 2. From any external process, call IUIAutomation::AddFocusChangedEventHandler 3. Observe: Excel UI becomes unresponsive during recalculation 4. Call RemoveFocusChangedEventHandler or RemoveAllEventHandlers 5. Observe: freezing persists Expected behavior: After unsubscription, Excel should stop processing focus change events on the UI thread and return to normal performance. Questions: 1. Is this a known issue? 2. Is there a programmatic way to reset Excel's internal UIA event state without restarting? Affected versions: Excel 2010, 2013, 2016, 2019, 2021, 2024

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Michelle-N 20,735 Microsoft External Staff Moderator

I would like to first clarify that this is a user-to-user support forum, and we are not Microsoft support. Moderators here do not have backend access and cannot directly intervene in Microsoft products or perform escalations. We can only provide technical guidance and best-practice recommendations based on reported issues.

Hi @Гудулин Виталий Дмитриевич

Based on your description, I understand that calling IUIAutomation::AddFocusChangedEventHandler from an external process forces Excel (versions 2010–2024) to execute formula recalculations directly on the UI thread. This causes severe unresponsiveness on large workbooks, and crucially, Excel fails to clean up its internal subscription state even after unsubscribing or killing the external calling process. The only current resolution is to restart Excel.

Here are the analysis and guidance based on technical research of Excel's UI architecture:

From a broader technical standpoint, AddFocusChangedEventHandler can act as a system-wide listener that introduces significant inter-process communication (IPC) overhead. Excel's UI thread is notoriously sensitive and heavily intertwined with its drawing and synchronous event-handling loops, especially when automatic recalculation is running on complex workbooks.

However, there is currently no official public documentation, Microsoft KB article, or developer release note classifying this specific unsubscription failure. I Since there is no official API method exposed to force Excel to flush its internal UIA event listener cache without a restart, you can try the following structural workarounds to reduce the performance impact:

Before executing your external UIA tool, programmatically or manually switch Excel to Manual Calculation mode (Application.Calculation = xlCalculationManual). This prevents Excel from triggering heavy recalculation chains on the UI thread every time a focus event fires. If possible, run the target workbook in a completely separate Excel instance so that if a freeze occurs or a restart is required, it does not interrupt the user's main Excel workspace.

To get this behavior analyzed by engineering teams, I highly recommend reporting this behavior directly to Github.
Thank you for your contribution and patience.

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Гудулин Виталий Дмитриевич 20 Reputation points

2026-06-06T07:40:48+00:00

Michelle-N, thank you for your comment.

Our agent for working with UIA creates a separate thread and initializes it as a COM MTA. In this thread, it subscribes to focus events via AddFocusChangedEventHandler, as well as unsubscribes via RemoveFocusChangedEventHandler and RemoveAllEventHandlers, which in the case of Excel do not help.

As for manually switching Excel to manual calculation mode — that won't work, because the agent starts at session startup and immediately enables focus tracking. By the time Excel launches, the focus handler is already connected, and Excel picks it up right away. And after Application.Calculation = xlCalculationManual, automatic recalculation will stop working for users in Excel.
Michelle-N 20,735 Reputation points Microsoft External Staff Moderator

2026-06-08T14:45:42.7666667+00:00

Hi @Гудулин Виталий Дмитриевич

Thank you for your update. After doing some research, due to the limitations of the environment and available resources, I would highly recommend that you raise this behavior directly on Github

so that subject-matter experts can assist you more effectively.
Гудулин Виталий Дмитриевич 20 Reputation points

2026-06-10T13:13:32.9666667+00:00

Thank you for the suggestion. Could you please point me to a specific repository? I wasn't able to find a public GitHub repository for Excel or Office bug reports. Should this be filed under a Windows UI Automation repository (e.g., microsoft/WindowsAppSDK or microsoft/accessibility-insights-windows), or is there a dedicated Office channel on GitHub you had in mind? Alternatively, would the Microsoft Feedback Portal be a more appropriate channel for an Excel-specific issue?
Michelle-N 20,735 Reputation points Microsoft External Staff Moderator

2026-06-10T14:43:15.0333333+00:00

Hi @Гудулин Виталий Дмитриевич

Thank you for your update. After conducting some research, I can raise this behavior through https://github.com/microsoft/Microsoft-UI-UIAutomation/issues. However, one option worth considering, as you mentioned, is the Microsoft Feedback Portal, where technical experts actively listen to user feedback. Thank you for your patience.