If you uncomment the line at the end (almost) of this playground script it will result in the “Some content text” being shown. I think this is a bug as it does not look like it needs to be resized and when resizeToFit is changed back to false the text still shows without problem?
A StackPanel needs the size (height or width depending on the orientation) of the child elements in pixel. resizeToFit sets the size in pixel, thats why the text is visible after setting this to true.
It is described here in the docs
Also you can see a warning about using percentage values in the browser console.
If you manually set the height in pixel it will also work:
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