Add PopupOverlayLayer

[MrJul]

What does the pull request do?

This PR adds a PopupOverlayLayer that's in front of OverlayLayer. Avalonia popups are now in PopupOverlayLayer and users are free to add controls to the OverlayLayer.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062551-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062581-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

I've amended the PR, ChromeOverlayLayer isn't added anymore.
#20770 adds a new API for custom-drawn chrome.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062831-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062889-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Copilot]

Pull request overview

Adds a dedicated PopupOverlayLayer positioned in front of OverlayLayer so Avalonia popups are hosted separately, leaving OverlayLayer available for user-defined overlays.

Changes:

• Introduce PopupOverlayLayer and register it in VisualLayerManager with a higher ZIndex than OverlayLayer.
• Update popup hosting (OverlayPopupHost) and related docs/tests to use PopupOverlayLayer instead of OverlayLayer.
• Expose OverlayLayer publicly to enable user code to add overlay content.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/Avalonia.Controls.UnitTests/ToolTipTests.cs	Updates overlay measurement hook to target PopupOverlayLayer.
src/Avalonia.Controls/Primitives/VisualLayerManager.cs	Adds PopupOverlayLayer and updates layer ZIndex ordering.
src/Avalonia.Controls/Primitives/TextSelectorLayer.cs	Refactors layer lookup logic; minor cleanup.
src/Avalonia.Controls/Primitives/PopupOverlayLayer.cs	New layer implementation for hosting popups above OverlayLayer.
src/Avalonia.Controls/Primitives/OverlayPopupHost.cs	Switches popup host creation/hosting to PopupOverlayLayer.
src/Avalonia.Controls/Primitives/OverlayLayer.cs	Makes OverlayLayer public and refactors lookup/measure logic.
src/Avalonia.Controls/Primitives/IPopupHost.cs	Updates XML docs to reference PopupOverlayLayer.
src/Avalonia.Controls/Primitives/AdornerLayer.cs	Makes AdornerLayer constructor public.

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[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062905-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]