Several of the themes shipped with Ubuntu have square button borders. Ubuntu's
default, Human, has rounded borders. This is fine in general, because rounded
borders look friendlier. However, it looks awkward and cluttersome in cases
where buttons are packed tightly against each other and/or a frame of something
else. These situations include:
* any buttons in the panel, including the "Show Desktop" button in the corner
of the screen
* the window switcher buttons
* toolbar buttons (see for example the "New" menubutton in Evolution)
* the buttons in CD Player
* the sheet navigation buttons in OpenOffice.org Spreadsheet.
The Show Desktop example is particularly bad, because the rounded border makes
the button look as if it doesn't include the highly accessible lower-left-most
pixel of the screen, when it actually does.
I suggest creating a themable distinction between buttons that are spaced apart
(and can therefore have rounded corners) and buttons that are in a confined
space (and should therefore always have square corners).
As a short-term solution, the Show Desktop button and window switcher buttons
could be special-cased to have square corners. (If the Show Desktop button is in
the screen's lower left corner, it could have no bottom or left borders at all.)
Well, I can confirm this. Buttons on edges does look like they don't include
pixels in the corrners, but they do. Designers? :)