When window is maximized, position of ui seemed to be adjusted by nsNativeThemeWin::GetWidgetPadding().
Bug 1542668 Comment 12 Edit History
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When window is maximized, position of ui seemed to be adjusted by nsNativeThemeWin::GetWidgetPadding().
GetWidgetPadding() has the following comment.
// XXX Maximized windows have an offscreen offset equal to
// the border padding. This should be addressed in nsWindow,
// but currently can't be, see UpdateNonClientMargins.
https://searchfox.org/mozilla-central/source/widget/windows/nsNativeThemeWin.cpp#2080