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The viewport size is the breakpoint width.

Note that you can also set the size of the dialog to full width. Then you do not have to worry about the size for smaller screens.

OK, I don't see a setup parameter in the JQuery to set the size to 100%

Where?

If the breakpoint is specified in the RWD properties) and the browser's viewport gets smaller than the specified value, the dialog will switch to full width mode.

Note:
Set the 'position childeren' property to floating, otherwise the content of the dialog will not be flexible.

But the Dialog is initially hidden and only open on a button click

So?

This does not matter.