fix(Input): clear focus on detached element when PresentationSource is torn down

[deeferentleeg]

Summary

Fixes #21759 — after clicking a menu item in a popup menu (e.g. "Delete"), the menu item is detached from the visual tree but keyboard focus stays on it, so shortcuts (Up arrow, Ctrl+Z, …) stop working until the user clicks another element.

Root cause

In src/Avalonia.Base/Input/InputElement.cs, OnDetachedFromVisualTreeCore clears focus with:

((FocusManager?)e.PresentationSource.InputRoot.FocusManager)
    ?.ClearFocusOnElementRemoved(this, root);

The ?. only guards the final ClearFocusOnElementRemoved call. When a popup closes, Popup.CloseCore() → popupHost.SetChild(null) detaches the menu items, and at that point the popup host's PresentationSource (or its InputRoot) may already be torn down. The direct chain e.PresentationSource.InputRoot.FocusManager is not null-safe on the intermediate accesses, so it either throws a NullReferenceException or silently skips ClearFocusOnElementRemoved — leaving KeyboardDevice.FocusedElement pointing at the detached element, which then has no visual parent to bubble key events to the window.

The fix

Use the existing FocusManager.GetFocusManager(this) helper, which already has a locator fallback (AvaloniaLocator.Current.GetService<IFocusManager>()) and is the pattern the Focus() method in this same file already uses. Null-guard the PresentationSource chain and fall back to GetFocusManager:

var focusManager = (FocusManager?)e.PresentationSource?.InputRoot?.FocusManager
    ?? FocusManager.GetFocusManager(this);
focusManager?.ClearFocusOnElementRemoved(this, root);

Minimal, no public-API change, and mirrors an established in-file pattern.

Notes

• I have not been able to run the full Avalonia build/test matrix locally; I'd welcome guidance on the preferred test for this (a headless focus/popup test) and am happy to add one.
• .NET Foundation CLA: please point me to the sign-in flow if one is required and I'll complete it before merge.

AI usage

An AI assistant was used to help locate the relevant code and draft this change. I have reviewed and understand every line of the diff, and I am the sole author of this contribution.

Fixes #21759

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067525-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[deeferentleeg]

@cla-avalonia agree