Currently, the functionality "Preview the app (F5)" allows users to preview the app for desktop mode. Considering the fact that app makers can create responsive layouts for canvas apps, it would be great if they have the flexibility to preview the app in various modes like
Tablet (Portrait) preview
Tablet (Landscape) preview
Mobile (Portrait) preview
Mobile (Landscape) preview
This would be in-line with the preview functionality available for Knowledge Articles in the Customer service model-driven app. Please see below.
It would save loads of time for app makers, to first publish the app, and then open the app on different devices to check if the controls in the app are aligning correctly based on the device.