## Scrolling a WPF TreeView or grouped ListBox in a VirtualizingStackPanel can cause a hang

### Scope
Major

### Version Introduced
4.5

### Version Reverted
4.5.1

### Source Analyzer Status
Planned

### Change Description

In the .NET Framework v4.5, scrolling a WPF
<xref:System.Windows.Controls.TreeView?displayProperty=name> in a virtualized
stack panel can cause hangs if there are margins in the viewport (between the
items in the <xref:System.Windows.Controls.TreeView?displayProperty=name>, for
example, or on an ItemsPresenter element). Additionally, in some cases,
different sized items in the view can cause instability even if there are no
margins.

- [ ] Quirked
- [ ] Build-time break

### Recommended Action

This bug can be avoided by upgrading to .NET Framework 4.5.1. Alternatively,
margins can be removed from view collections (like
<xref:System.Windows.Controls.TreeView?displayProperty=name>s) within
virtualized stack panels if all contained items are the same size.

### Affected APIs
* `M:System.Windows.Controls.VirtualizingStackPanel.SetIsVirtualizing(System.Windows.DependencyObject,System.Boolean)`

### Category
Windows Presentation Foundation (WPF)

[More information](http://connect.microsoft.com/VisualStudio/feedback/details/763639/wpf-application-built-on-net-4-0-freezes-under-system-with-net-4-5-while-scrolling-the-<xref:System.Windows.Controls.TreeView?displayProperty=name>-under-specific-conditions)

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