Bug: Expander Should Not Have a MaxWidth

[robloo]

Describe the bug

Expander has a MaxWidth set in the template:

microsoft-ui-xaml/dev/Expander/Expander.xaml

Line 26 in 4208a01

<Setter Property="MaxWidth" Value="{ThemeResource FlyoutThemeMaxWidth}" />

This prevents HorizontalAlignment = "Stretch" from working (which is also set by default in the style). Instead, just like other controls, MaxWidth should be unset (double.NaN) and HorizontalAlignment = "Left". I can't think of any other content control that constrains width like this.

Steps to reproduce the bug

1. Add a new Expander control to a page
2. The expander will not stretch past the MaxWidth

Expected behavior

Expander should stretch as most all other controls do.

Screenshots

Version Info

NuGet package version: 2.6.0 preview

Windows app type:

UWP	Win32

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Xbox
Surface Hub
IoT

Additional context

[michael-hawker]

🦙 Related to #4248.