Learn how to set (and unset) a default layout, so that the window for selecting a layout does not appear each time you start a new chapter.
When you add a new chapter or subchapter, the Layout selection window will appear.
1. Check Don't ask again in the bottom left corner of the window.
2. Select your preferred layout (reflowable or fixed).
3. Your new chapter will be created.
Next time you create a new chapter it will instantly appear with your prefered layout.
Remove your default setting
After you set a preferred layout, a button with 3 dots ... appeared to the right of the add New chapter button. If you later on decide to change the default layout mode, you may click on the button with the three dots and the layout selection window will open for you. You may then set a new default, or simply add a new chapter with either layout.