NSMenuItem.setImage stretches the row to the leading image's pixel
size regardless of the surrounding rows, so any non-empty bitmap on
the About entry made it visibly taller than the rest of the tray
menu — leaving 16, 18 or 22 px versions all looking wrong next to
the unadorned rows above and below.
Drop the macOS brand mark and gate the SetBitmap call on a non-empty
byte slice; iconMenuNetbird is now nil on macOS, so the About row
falls back to text only. Windows and Linux still ship the brand mark
through their per-platform embed files.
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