On macOS Tahoe (26+), the default title bar is 32px with 14px buttons,
so the old formula (button_height + PAD_Y = 14 + 18 = 32) produced no
change. Add TITLEBAR_EXTRA_HEIGHT to push the title bar taller than
the Tahoe default. Use OnceLock to capture the original default height
so repeated calls don't accumulate extra pixels.
Also update CI runners to macos-26 for Tahoe SDK builds and adjust
frontend padding for larger stoplights.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
