macOS Tahoe – Window maximise leaves phantom Dock space on multiple DisplayLink displ

[holtond]

My first posting to the forum with some details of what I'm seeing and think it's a bug (or at least unexpected/unwanted behaviour). Would be good if anyone else is able to confirm and/or offer advice.

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Title: macOS Tahoe – Window maximise leaves phantom Dock space on multiple DisplayLink displays

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Summary:
When using multiple DisplayLink-connected monitors, macOS incorrectly reserves Dock space on more than one display. This results in windows not fully maximising when using double-click on the title bar, leaving a gap at the bottom of the screen where no Dock is present.

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Environment:
- Device: MacBook Pro 16-inch (Nov 2023) – Apple M3 Pro
- OS: macOS Tahoe 26.4
- DisplayLink version: 15.1.0
- Docking station: Dell D6000
- External monitors: 2 × Dell U2415 (connected via DisplayLink)
- Internal display: Open and active (3rd display)
- Primary display: One of the DisplayLink monitors

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Steps to Reproduce:

1. Connect two external monitors via DisplayLink (Dell D6000).
2. Set one of the DisplayLink monitors as the primary display (menu bar present).
3. Ensure the macOS Dock is visible on one DisplayLink display.
4. On the other DisplayLink display (without the Dock visible), open any application window.
5. Double-click the window title bar to maximise (zoom).

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Expected Result:
The window should expand to fill the full usable area of that display (i.e. no reserved space at the bottom, since the Dock is not present on that screen).

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Actual Result:
The window expands but leaves a blank gap at the bottom of the screen, as if space has been reserved for a Dock that is not visible on that display.

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Additional Observations:

• The issue only occurs when the Dock is positioned on a DisplayLink screen and maximising a window on another DisplayLink screen.
• The green window button (full screen/zoom) does not exhibit this issue and works as expected.
• Toggling Dock position, changing primary display, and restarting DisplayLink Manager do not resolve the issue.
• Behaviour suggests macOS is incorrectly reporting or calculating Dock insets across multiple DisplayLink displays.

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Impact:

• Reduces usable screen space on affected displays.
• Creates inconsistent window management behaviour depending on maximise method.
• Affects usability in multi-monitor workflows.

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Notes:
This appears to be a regression or compatibility issue with macOS Tahoe and DisplayLink multi-display handling, specifically around system UI insets (Dock positioning).

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Request:
Please investigate how DisplayLink virtual displays report Dock/safe area insets to macOS, as it appears multiple displays are being treated as having a Dock simultaneously.

[Szymon]

Hi holtond,

This is a very detailed report. Since this behavior isn't consistently reproducible on other macOS Tahoe/DisplayLink configurations (at least on our side), it suggests a localized WindowServer geometry mismatch where macOS is 'remembering' a Dock footprint on a screen where it no longer exists.

• Check Mission Control Settings: Ensure that 'Displays have separate Spaces' is enabled in System Settings > Desktop & Dock. If it is already ON, try toggling it OFF, logging out, and then toggling it back ON. This forces macOS to recalculate the 'Available Area' for each individual monitor.
  
• The 'Dock Flush' Technique: Change the Dock position to Left or Right (in System Settings > Desktop & Dock), then change it back to Bottom. This often forces the window manager to 'release' reserved space on secondary monitors.
  
• Primary Display Assignment: In System Settings > Displays > Arrangement, try dragging the white 'Menu Bar' icon to a different screen and then back to your preferred primary display. This reset often resolves 'phantom' UI gaps.

I hope some of it helps.

Regards,
Szymon

[holtond]

Hi Szymon,

Thanks for the reply and suggestions, unfortunately none seem to have helped.

I understand there's a new release available so going to upgrade to that and see if it helps - have already updated to macOS 26.4.1 with the existing DisplayLink version without any change.