Fixes #8293

Description

When a TabItem is selected in TabControl, it sets focus to its content (unless the TabItem itself is focused). If the content gets succesfully focused, then the TabItem supresses receiving focus to not steal it from the content back.

When the content is a Button in its own focus scope (such as in a ToolBar), the Button will try to switch back to the main focus scope after pressed (on losing mouse capture).

Finally, when a TabItem is focused, the TabControl will switch to that tab.

As a result, if a TabItem contains ToolBar with a Button, that Button will try to re-focus the previously focused element. However, if the TabItem suppressed receiving focus, then the previously selected tab will still be remembered as the last focused element, and re-focusing it will select it and thus unexpectedly change the displayed tab.

Customer Impact

Customers not taking this fix will experience undesirable UI behaviour with buttons in their own focus scope inside a TabItem. By default that is only the case for buttons inside ToolBar, although developers can change that.

Workarounds of the original issue exist, as described in #8293 (comment)

Regression

No. Well technically the Switch.System.Windows.Controls.TabControl.SelectionPropertiesCanLagBehindSelectionChangedEvent compatibility switch introduced this issue in .NET 4.7.1. Before that, the TabItem would not opt out of receiving focus, so this issue did not happen, but there were even worse problems.

Testing

Compiled and tested with the repro app in #8293 on .NET 8.0.100-preview.7.23376.3. Verified the issue repros without the fix and does not repro after the fix.

Risk

This is potentially a higher impact change (focusing behaviour changes) and there is a lot of corner cases that might be difficult to pick up. To minimize the risk as much as possible, the new behavior is executed in a very specific case only:

1. Selecting the tab item did not change the focused element.
2. The focused element is another tab item of the same control. (in that case the code does not run, so issue does not exist)
3. The tab item succeeded focusing its content and suppressed receiving focus.
4. The newly focused element is in a different focus scope.

Microsoft Reviewers: Open in CodeFlow