Windows 11 25H2: "There is already a toolbar hidden on this side of the screen" Error on Triple Monitors

Concise answer​

The message “There is already a toolbar hidden on this side of the screen” is coming from the legacy taskbar auto‑hide subsystem, not from ExplorerPatcher, StartAllBack, or any third‑party tool.It means Windows believes another auto‑hidden taskbar already occupies that screen edge, so it forcibly disables auto‑hide on the primary taskbar.

On 25H2, this is almost always caused by a phantom taskbar registration created during monitor topology changes—especially in multi‑monitor or simulator environments.

What the message actually means
Windows tracks taskbars using AppBar registrations (the same API used since Windows 95).Each taskbar—real or phantom—registers itself with:

• a monitor
• an edge (top/bottom/left/right)
• an auto‑hide flag

If Windows thinks an edge is already claimed, it throws the error you saw.

On 25H2, a regression causes:

• stale AppBar entries to persist after monitor layout changes
• “ghost” taskbars to remain registered on edges that no longer exist
• auto‑hide to be forcibly turned off when Explorer detects a conflict

This is why the checkbox unchecks itself on every boot.

Why it appears on the middle monitor​

Your middle display is almost certainly the logical primary in Windows’ coordinate system.When Explorer starts:

1. It enumerates AppBars
2. It finds a phantom one registered on that monitor
3. It tries to register the real taskbar
4. The API returns ABN_STATECHANGE / ABERR_ALREADY_REGISTERED
5. Explorer shows the message and disables auto‑hide

This is consistent with:

• triple‑monitor setups
• simulators that present virtual displays
• 25H2’s new taskbar code path

How to fix it
There are three reliable fixes, in order of least to most invasive.

1. Reset all AppBar registrations (recommended)​

This clears the phantom taskbar entries.

1. Open Regedit
2. Navigate to:
   
   Code
   
   HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\AppBarData<br>
3. Delete all values under AppBarData (not the key itself)
4. Restart Explorer.exe or reboot

This forces Windows to rebuild taskbar registrations cleanly.

2. Toggle monitor topology to force a rebuild​

This works because Windows flushes AppBars when the display layout changes.

1. Open Settings → System → Display
2. Temporarily disable one monitor
3. Apply
4. Re‑enable it
5. Reboot

This often clears the phantom entry without registry edits.

3. Full Explorer taskbar reset​

If the above fails:

1. Delete these keys:
   
   Code
   
   HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\StuckRects3<br>HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\StuckRects2<br>
2. Reboot

This resets taskbar position, auto‑hide state, and monitor bindings.

Why it started after 26200.8457​

Build 26200.8457 introduced changes to:

• multi‑monitor taskbar placement
• auto‑hide behavior
• AppBar registration timing during Explorer startup

These changes exposed a long‑standing bug where stale AppBar entries weren’t cleaned up properly.

Your simulators likely trigger this more often because they:

• create/destroy virtual monitors
• change EDIDs
• alter display topology during boot

All of these generate the exact conditions that produce phantom AppBars.

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