SelectingItemsControl overwrites its own `SelectedIndex` value at startup

[TomEdwardsEnscape]

Describe the bug

#20798 introduces a change in behaviour which breaks our application. We want the default selected item of a SelectingItemsControl to be a hidden item at index 0, but now the first visible and enabled index is forcibly selected during startup.

It's legal to have a hidden item selected: we can still programatically select the hidden item at index 0. But now, Avalonia picks a fight with us about this when the control is first created.

The new behaviour occurs even if SelectedIndex="0" has been specified.

To Reproduce

<TabStrip SelectedIndex="0">
  <TabStripItem IsVisible="False" Content="HELLO"/>
  <TabStripItem Content="WORLD"/>
</TabStrip>

The selected item will be "WORLD", even though I specified that it should be "HELLO".

You can re-select index 0 after the TapStrip has fully initialised, but at this point the side-effects of selecting index 1 have already occurred.

Expected behavior

The first item of a SelectingItemsControl is selected by default even if that item is hidden or disabled. Don't try to behave as if the user interacted with the control, they didn't!

If this new behaviour really is the intent, than at the very least the value of SelectedIndex should be respected so that the initial auto-selection of a visible+enabled item can be avoided.

Avalonia version

12.1.1

OS

No response

Additional context

No response

[timunie]

There may be rare usecases to select an invisble or hidden item on startup. In general this is not a good UX in gernal, but again, there may be situation where it is needed. Luckily the new behavior can be changed easily. One "simply" needs to change the SelectionMode to Single. Since for TabControl it's a protected property, the solution is as follows:

public class TabStripEx : TabStrip
{
    protected override Type StyleKeyOverride => typeof(TabStrip);

    static TabStripEx()
    {
        SelectionModeProperty.OverrideDefaultValue(typeof(TabStripEx), SelectionMode.Single);
    }
}

<local:TabStripEx SelectedIndex="0">
  <TabStripItem Content="Disabled" IsEnabled="False"/>
  <TabStripItem Content="Invisible" IsVisible="False" />
  <TabStripItem Content="Different One" />
</local:TabStripEx>

Hope that it unblocks you for now.