Fix TabItem's Content inheriting TabControl's DataContext instead of TabItem's

[ptasev]

What does the pull request do?

Fixes the TabItem's Content inheriting TabControl's DataContext instead of TabItem's.

What is the current behavior?

When the TabItem content's DataContext is not explicitly set, it inherits the DataContext from TabControl after UpdateSelectedContent.

1. TabItem's Content inherits DataContext from TabItem after getting added to logical tree (TabControl's Items)
2. TabItem's Content has its InheritanceParent set to TabControl.ContentPart (ContentPresenter) when TabItem is selected.
3. This makes the TabItem's Content DataContext effectively inherit TabControl's DataContext

What is the updated/expected behavior with this PR?

The TabItem's Content keeps its DataContext inherited from TabItem.

How was the solution implemented (if it's not obvious)?

First, the initial UpdateSelectedContent call is changed to happen in ContainerForItemPreparedOverride rather than PrepareContainerForItemOverride so that the TabItem Content's DataContext can be inherited from TabItem after getting added to the logical tree.

Then, when UpdateSelectedContent is called the original Content's DataContext is saved to a variable before changing its InheritanceParent to ContentPart. The saved DataContext is reapplied after. See code comments for further details.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Perhaps changing the expectation of UpdateSelectedContent being called from ContainerForItemPreparedOverride instead?

Obsoletions / Deprecations

Fixed issues

Fixes #10958

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0061713-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Could you please explain why this was moved to ContainerForItemPreparedOverride? The DataContext should probably be set asap and PrepareContainerForItemOverride seems like the right place for that.

Reverting this still makes the new test passes so I'm a bit confused.

[ptasev]

You're right, I must have gotten confused in my attempts to understand what's happening. Good catch!

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0062113-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM, thank you!