I found my x and Y values (shown in the advanced tab/pane) where off so this may be related to why I am seeing this strange behaviour. I would be curious to know if others who see their width go to zero if the X and Y data card postions where a factor in this problem.
I was having this happen to me as well (but not every time). What I've found is if I rename the Form after I've connected the form to a data source, the issue will occur where trying to resize the data cards with the mouse will set the width to zero. If I rename the form first or not rename it all, the resizing works fine.
I am also experiencing that data cards randomly change position in forms, especially horizontal forms. I am using both relative positioning with formulas in the X,Y, W and H fields as well as hard coding the positions.
This is a bug. Here is a workaround: Save the app, publish it, close it and then reopen in Edit. Then I was able to resize. Seems to happen after you rename the form. Get it saved then reopen and you're good.
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