Don't enter a KeyboardNavigationMode.None group via its active element

[ethanstoner]

Fixes #22048.

Split into the two commits the contributing guide asks for: the failing tests first, then the fix.

Root cause

GetNextTab and GetPrevTab fall back to TabOnceActiveElement when entering a container, and neither checked the navigation mode first:

if (e == null)
{
    if (IsTabStop(container))
        return container;

    // Using ActiveElement if set
    var activeElement = GetActiveElement(container);
    if (activeElement != null)
        return GetNextTab(null, activeElement, true);
}

ItemsControl.OnGotFocus records the last focused child as that active element:

// If the focus is coming from a child control, set the tab once active element to
// the focused control. This ensures that tabbing back into the control will focus
// the last focused control when TabNavigationMode == Once.
if (e.Source != this && e.Source is IInputElement ie)
    KeyboardNavigation.SetTabOnceActiveElement(this, ie);

So once a ListBox item has been focused, the ListBox has an active element pointing inside itself, and Tab enters it even with TabNavigation="None". GetNextTabInGroup already returns null for None, so the group walk was correct; only this fallback leaked.

This also explains the asymmetry in the report. TreeView is not inherently better behaved here, it just had no focused item in the repro and therefore no active element. A TreeView whose item has been focused should show the same bug.

The fix

A None group never hands out an element inside itself, not even a remembered one. GetNextTab skips the active-element lookup for None and falls through to the existing group walk, which already returns nothing for None; GetPrevTab does the same.

I deliberately left IsTabStop(container) above it alone. None governs navigation within the group, and a container that is itself a tab stop should still be reachable, which matches WPF. Changing that would be a wider behavioural change than this issue calls for.

Tests

Two new tests in KeyboardNavigationTests_Tab, one per direction. Both fail on the first commit and pass on the second:

failed ...KeyboardNavigationTests_Tab.Next_None_Skips_Container_With_Active_Element_Inside
failed ...KeyboardNavigationTests_Tab.Previous_None_Skips_Container_With_Active_Element_Inside
  total: 3440   failed: 2   succeeded: 3426

after the fix:

  total: 3440   failed: 0   succeeded: 3428   skipped: 12

Worth noting why this was not already covered: the existing Next_None_Skips_Container does call SetTabOnceActiveElement, but points it at a button in a sibling panel, which is also the expected result, so the assertion passes whether or not the fallback fires.

Avalonia.Controls.UnitTests as well, since this is a shared navigation path: 3915 total, 0 failed, 3914 succeeded, 1 skipped.

Windows, .NET 10, Release.

Not verified

I have not run the attached sample app, only the unit tests, so I have not confirmed the end-to-end fix in a real window. I also have not checked whether TreeView reproduces the same bug once one of its items has been focused, though the analysis above says it should.

AI usage

Per the contributing guide, which accepts AI-assisted contributions but wants a human in the loop: Claude Code (Claude Opus 5) helped locate the cause and draft this. I reviewed the diff and ran all the test runs above myself, and will handle review comments personally.

[avaloniaui-bot]

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[grokys]

I have not run the attached sample app, only the unit tests, so I have not confirmed the end-to-end fix in a real window

🤦‍♂️

[grokys]

This code comes directly from WPF:

https://github.com/dotnet/wpf/blob/360950b3ceb503d68bbb2fbc2ab9271e4cda90a5/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Input/KeyboardNavigation.cs#L2090-L2093

Does this issue also reproduce on WPF?

[ethanstoner]

No, it doesn't reproduce on WPF, and the reason is that WPF gates the write rather than the read.

KeyboardNavigation.UpdateActiveElement only records an active element for Once groups:

// Update ActiveElement only if container has TabNavigation = Once
if (GetKeyNavigationMode(container) == KeyboardNavigationMode.Once)
{
    SetActiveElement(container, activeElement);
}

https://github.com/dotnet/wpf/blob/360950b3ceb503d68bbb2fbc2ab9271e4cda90a5/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Input/KeyboardNavigation.cs#L948-L960

That's the only write path: SetActiveElement is private with a single caller, and SetTabOnceActiveElement is only called from it. So in WPF a None container never has an active element, and the code you linked never gets the chance to follow one.

Avalonia records it unconditionally, in three places: ItemsControl.OnGotFocus (ItemsControl.cs:543), and the selection anchor in SelectingItemsControl.cs:539 and :971.

The selection path is the one in the report. The ListBox has an active element as soon as an item is selected, without ever being focused. That is also why the reporter's TreeView looked correct: it had no selected item, so nothing recorded one.

[grokys]

Again, this code is directly from WPF:

https://github.com/dotnet/wpf/blob/360950b3ceb503d68bbb2fbc2ab9271e4cda90a5/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Input/KeyboardNavigation.cs#L2266

I'd like to understand why we'd want to diverge from WPF.

[ethanstoner]

Agreed that matching WPF should be the default, so I tried WPF's approach first: gate the write the way UpdateActiveElement does and leave both of these hunks untouched.

It doesn't survive here. Avalonia uses TabOnceActiveElement for a second job that WPF doesn't give it, as the VirtualizingStackPanel focus anchor (VirtualizingStackPanel.cs:1140 and :1386).

Gating the write on Once broke that. Avalonia.Controls.UnitTests went from 3915 passed / 0 failed to 11 failures, all VirtualizingStackPanel focus tests (Focusing_Another_Element_Recycles_Original_Focus_Element, Focused_Element_Outside_Realized_Range_Is_Not_Arranged_In_Viewport and similar).

So the divergence is forced by that property carrying two responsibilities in Avalonia. The read side was where I could fix it without disturbing virtualization.

The cleaner long term option is to split the two concerns, a navigation active element and a separate virtualization anchor. Then Avalonia could take WPF's write gate verbatim and both of these hunks would go back to matching WPF exactly. That's a bigger change and your call, and I'm happy to do it that way if you'd prefer.

[ethanstoner]

Fair point on the sample app. I've added a ListBox level test that goes through the path in the report rather than setting the active element by hand: a ListBox with TabNavigation="None" and a selected item, where the active element is recorded by selection rather than by focus.

It fails on main with Assert.Same() Failure: Values are not the same instance and passes with the change. Avalonia.Controls.UnitTests is 3916 passed / 0 failed, Avalonia.Base.UnitTests 3440 passed / 0 failed.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069473-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[grokys]

@ethanstoner is your human there?