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By Default, when you add a Child Height, this will be set to Desktop Only, meaning it will not apply to Mobile. This is the default behaviour since a Mobile is a smaller screen than Desktop, and therefore the Child elements most often will fill the screen, and therefore no height is required.

However in some cases, the Child elements on Mobile do not fill the screen, e.g. Title Bars. In which case, you can set Fixed Height Devices to 'All Devices' to ensure you Height will also display.

DESIGNER TIPS:

If your Parent Fixed Height does not show on Mobile:
Click on the Parent Frame and set Fixed Height Devices to 'All Devices'.

If your Child Frame does not align on Mobile:
Click on the Child Frame and set Child Height Devices to 'All Devices'.