This is a well documented and conversed issue in this forum.
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Thunkable isn’t using a flex/responsive environment outta the box. More like stretch and scale.
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iOS is a buncha haterz just kidding BUT Apple is THE reason that white bar is there and Thunkable is simply playing by the rules albeit annoying as they are
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check this post from @matt_conroy
Layout changes in the downloaded app vs. preview in Thunkable app - #2 by matt_conroy