===== Mar 26, 2021 updated ==============
Using vertical screen layout is supported in below build:

EDIUS 9.55.7303 or later – Latest build is here
EDIUS 10.20.7490 or later – Latest build is here

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Mask filter dialog has ‘Multi’ option that is active if your Windows is multiple monitor environment. Once you click ‘Multi’ button, EDIUS will expand Mask dialog to use both monitors.
However, the option only considers working EDIUS in horizontal multiple monitor environment.
As you may know, Windows allows free layout of connected monitors. So, for example, if you build vertical multiple monitor environment, Multi option doesn’t work correctly. Under the environment, if you click Multi option button, mask dialog is hidden. In this case, press Alt + f.4 key to close the dialog and do not use the option (or change monitor layout to be horizontal multiple monitor environment)