> My experience is that the Windows windowing system supports keyboard navigation through the UI elements much, much more than Apple (and often Ubuntu, too).
That's due to historical reasons. While both X11 and Apple could assume the presence of a mouse, it was an optional add-on for a PC back in the MS-DOS days; the original 16-bit Windows had to be fully usable even in the absence of a mouse (the only thing which came with Windows and didn't work without a mouse was the image editor Paintbrush). A lot of that keyboard-oriented legacy still survives in the Windows API.
That's due to historical reasons. While both X11 and Apple could assume the presence of a mouse, it was an optional add-on for a PC back in the MS-DOS days; the original 16-bit Windows had to be fully usable even in the absence of a mouse (the only thing which came with Windows and didn't work without a mouse was the image editor Paintbrush). A lot of that keyboard-oriented legacy still survives in the Windows API.