Use the "mixed-aspect-ratio" grid style:
http://sdkdocs.roku.com/display/RokuSDK ... tringstyleThe documentation for setting the row aspects is wrong, though. You specify what aspect each row should be with the SetListPosterStyles(listStyles as Array) method. It accepts an array of strings with the values of either "portrait" or "landscape". The array should be aligned with the array used to define the rows.
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