Thingworx Widget Extension for Container Mashup

I have a widget extension that was developed when Mashups used Layout instead of Flex Container.
My extension is OpenLayers showing geographic data from a database as a map. Its features are ok. However, in mashups that use Flex Container instead of Layout, the map (widget) starts with the disproportionate height and width and only after it stretches the page size (maximizing the browser, changing the screen size through the inspect element. ..) that the proportions are organized.
Is there a class I should add to the div of this widget or something else that differs in the development of Widget Extension with Mashups that use Layout for those that use Flex Container?

See attached print screen.