The non client parts of the window are not scaled per-monitor by default on windows, so the title bar is either too big or too small when the window is not on the primary display. Microsoft have added a new 'EnableNonClientDpiScaling' function call to fix this (See: https://msdn.microsoft.com/en-us/library/windows/desktop/mt748621(v=vs.85).aspx).
It only affects top level windows. Dialogs can be fixed by selectively using 'system' DPI awareness as firefox does (See: https://hg.mozilla.org/mozilla-central/rev/1b782dcb677a)