The header toggle's PanelLeftOpenIcon was size-5 while the sidebar's
close icon was size-4 — the button visibly grew when the sidebar closed.
The small look is the standard: both glyphs are size-4 in identical
size-9 boxes now.
Live: open glyph 16px, close glyph 16px, button boxes 36px both states.
Gates: typecheck, lint, 469/469.