WindowGlassEffect.apply() replaced the window's contentView with the
glass/blur view on all macOS versions. On macOS 13-15, swapping the
contentView on a window with fullSizeContentView + transparent titlebar
breaks the titlebar view hierarchy, causing traffic light buttons to
disappear behind the content.
Split into two paths:
- macOS 26+ (NSGlassEffectView): full contentView replacement (safe)
- macOS 13-15: insert NSVisualEffectView as a background subview of the
existing contentView, preserving the titlebar hierarchy
Also fix the custom titlebar overlay overlapping the traffic lights and
left titlebar accessories (sidebar/bell/+ buttons) when the sidebar is
hidden. Use TitlebarLeadingInsetReader to dynamically measure the actual
width of traffic lights + left accessories and apply it as leading padding.
fe0b33f19b