contain:strict includes size containment which caused a timing issue
where the flex-based main-panel height wasn't resolved before the
virtualizer measured its container, resulting in track-list rendering
at ~20% height on first load until something triggered a relayout.
- contain: layout style on .content-area to isolate main+queue from header/sidebar
- contain: strict on .main-panel for maximum layout isolation
- contain: layout style paint on .main-panel > * for paint containment per view
- contain: layout style paint on sidebar to isolate from main panel
- contain: layout style on .bottom-bar to isolate footer from content reflows