When opening the QFontComboBox the width is larger than the control.
See the following image.
I assume is not an intended behavior..

On our application we see the problem appearing in Windows, Mac, Wasm. 
You can easily replicate with the example "Standard Dialogs".
Aafter line 179
  layout->addWidget(multiLineTextLabel, 4, 1);

add the following

QFontComboBox *fontComboBox =
        new QFontComboBox();

layout->addWidget(fontComboBox,5,1);