I created a canvas app to try the new responsive containers following the amazing and great walkthrough video by @RezaDorrani .
You can find it here: Power Apps Responsive Containers & Screen Templates
I have followed the same steps mentioned in the video and tested the app in preview mode by shrinking the size of the browser.
After I have published the app and play the app on the web browser the gallery width is very small knowing that its width property is "640" and the MinimumWidth property is "250" also the screen size is large.
Whereas in the preview mode using the "Windows size" and "Canvas size" the width of the gallery is working perfectly.
Please have a look at the difference between the preview mode, the play mode, and a screenshot showing the properties of the gallery:
Any idea why in the play mode it's not showing the same width size as in the preview mode for the gallery?
Any help will be greatly appreciated.