ComboBox will not show dropdown while in a TextBox InnerLeftContent

[sunwayking]

Describe the bug

I put a ComboBox inside TextBox.InnerLeftContent. The ComboBox dropdown didn't show itself when I click on the ComboBox.

To Reproduce

<TextBox VerticalContentAlignment="Center" Watermark="http://" IsHitTestVisible="True">
    <TextBox.InnerLeftContent>
        <ComboBox Margin="4" Width="100" AutoScrollToSelectedItem="True"  IsHitTestVisible="True"
                  ItemsSource="{Binding ProtocolPrefixes}"
                  SelectedIndex="{Binding SelectedProtocolPrefixIndex}">
            <ComboBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ComboBox.ItemsPanel>
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding}" />
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </TextBox.InnerLeftContent>
    <TextBox.InnerRightContent>
        <StackPanel Orientation="Horizontal" Spacing="4" Margin="4">
            <Button Content="POST" />
            <Button Content="GET" />
        </StackPanel>
    </TextBox.InnerRightContent>
</TextBox>```

### Expected behavior

The ComboBox that is inside TextBox.InnerLeftContent should show its dropdown content, when clicking on it.

### Avalonia version

11.2.2

### OS

_No response_

### Additional context

_No response_

[stevemonaco]

The ComboBox doesn't receive OnPointerReleased which is where IsDropDownOpen is changed to display the dropdown. This is because TextBox.OnPointerPressed captures the mouse. This TextBox event happens because ComboBox.OnPointerPressed doesn't handle the event, even when it's determined that the :pressed pseudoclass is to be set.

Avalonia/src/Avalonia.Controls/ComboBox.cs

Lines 309 to 312 in ab1d77a

	else
	{
	PseudoClasses.Set(pcPressed, true);
	}

I don't know of a reasonable workaround until the bug is fixed.

Repro without bindings:

<StackPanel>
    <TextBox
        VerticalContentAlignment="Center"
        IsHitTestVisible="True"
        Watermark="http://">
        <TextBox.InnerLeftContent>
            <ComboBox
                Width="100"
                Margin="4"
                AutoScrollToSelectedItem="True"
                IsHitTestVisible="True">
                <TextBlock Text="https" />
                <TextBlock Text="http" />
                <TextBlock Text="ftp" />
            </ComboBox>
        </TextBox.InnerLeftContent>
        <TextBox.InnerRightContent>
            <StackPanel
                Margin="4"
                Orientation="Horizontal"
                Spacing="4">
                <Button Content="POST" />
                <Button Content="GET" />
            </StackPanel>
        </TextBox.InnerRightContent>
    </TextBox>

    <ComboBox
        Width="100"
        Margin="4"
        AutoScrollToSelectedItem="True"
        IsHitTestVisible="True">
        <TextBlock Text="https" />
        <TextBlock Text="http" />
        <TextBlock Text="ftp" />
    </ComboBox>
</StackPanel>

[sunwayking]

@stevemonaco Thanks for helping me to figure out why inner combobox not showing its dropdown.
However a workaround is still needed.

[stevemonaco]

There's a pretty hacky workaround with event subscriptions. Building from the example above, naming the TextBox, and going into code-behind:

public MainWindow()
{
    InitializeComponent();

    var left = textbox.InnerLeftContent as Control;
    left?.AddHandler(PointerPressedEvent, InnerLeftContent_PointerPressed, RoutingStrategies.Bubble);
}

private void InnerLeftContent_PointerPressed(object? sender, PointerPressedEventArgs e)
{
    e.Handled = true;
}

Handling the event here will not allow the TextBox to capture the mouse when clicked on the InnerLeftContent area (assuming hit test succeeds, of course). A proper fix would require a change to the Avalonia code base.