I'm not the developer of PageHeader, but I had a look at the code, and it seems that's the expected outcome.
When PageHeader.flickable is set, PageHeader adds its height to the topMargin to ensure all the content is visible.
In your case, when PageHeader.flickable is set both PageHeader and you are setting topMargin. As a consequence, you get a double top margin.
DISCLAIMER: I just had a quick look at the code, so take my words with a pinch of salt :)
If, instead, you get the double height also when PageHeader.flickable is null, then my words are void.
I'm not the developer of PageHeader, but I had a look at the code, and it seems that's the expected outcome.
When PageHeader. flickable is set, PageHeader adds its height to the topMargin to ensure all the content is visible.
In your case, when PageHeader. flickable is set both PageHeader and you are setting topMargin. As a consequence, you get a double top margin.
DISCLAIMER: I just had a quick look at the code, so take my words with a pinch of salt :)
If, instead, you get the double height also when PageHeader. flickable is null, then my words are void.