Fix TreeView auto-expansion when opening trace files

[Copilot]

When a user double-clicks on a trace file in the main window, PerfView should automatically expand the first level of options for that trace file. However, the TreeView was not auto-expanding because the IsExpanded property binding was missing from the TreeView's ItemContainerStyle.

Root Cause

The data model (PerfViewTreeItem) correctly sets IsExpanded = true when files are opened (in PerfViewData.cs lines 535 and 10689), but the TreeView's ItemContainerStyle in MainWindow.xaml was missing the binding to connect this property to the UI:

<TreeView.ItemContainerStyle>
    <Style TargetType="TreeViewItem">
        <Setter Property="AutomationProperties.Name" Value="{Binding Name}" />
        <!-- Missing: IsExpanded binding -->
    </Style>
</TreeView.ItemContainerStyle>

Solution

Added the missing IsExpanded property binding to the TreeView's ItemContainerStyle:

<TreeView.ItemContainerStyle>
    <Style TargetType="TreeViewItem">
        <Setter Property="AutomationProperties.Name" Value="{Binding Name}" />
        <Setter Property="IsExpanded" Value="{Binding IsExpanded}" />
    </Style>
</TreeView.ItemContainerStyle>

This connects the data model's IsExpanded property to the TreeViewItem's IsExpanded property, enabling automatic expansion when trace files are opened.

Change Impact

• Minimal: Only 1 line added, 0 lines removed
• Non-breaking: No existing functionality is affected
• Standard pattern: Uses standard WPF property binding approach

Fixes #2217.

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[brianrob]

Copilot post-mortem:

• I know very little about WPF, and so this would have been a heavier lift for me.
• All I had to do was a bit of testing locally to see that this worked.

[mconnew]

[marklio]

I was initially annoyed that I had to go try to discover the same things that Copilot did in order to fix it, but I remembered I could look at the session and see what it learned. This was super-helpful in confirming that PerfViewTreeItem has the IsExpanded property backed with an INotifyPropertyChanged implementation that seemed wired-up correctly. Still, I wish Copilot had a summary that included the info "The tree view is backed by PerfViewTreeItem objects that have an IsExpanded property on them"