Skip hidden TitleBar children in visitChildrenForSemantics

[melo936]

Problem

_TitleSubtitleOverflowBox hides the title or subtitle when they no longer fit. Hidden children are skipped in performLayout, paint, and hit testing, but the default visitChildrenForSemantics still walks every child. A hidden child that was marked dirty by the size change is never laid out, so the semantics walk hits the assert. Minimizing a maximized window with a screen reader running triggers the _needsLayout assert.

Special thanks to @Stasium for helping.

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation