KVADRANT_PASSWORD_MASK
What a masked character is drawn as, and it is not Silverlight's.
PasswordChar defaults to U+25CF BLACK CIRCLE, and Selawik's cmap does not carry it — read out of the font, not assumed. A text field draws through BasicTextField, which does no script splitting, so the circle fell through to whatever the operating system had: the dots were a different glyph on every platform, and the way it surfaced was a screenshot that would not match between two of them.
U+2022 BULLET is in Selawik. It is smaller than the circle and it is this project's choice rather than Microsoft's, which is why it is a parameter of KvadrantPasswordBox — a caller with a font that has the circle can ask for it back.