There is an extra long indentation to the left before the first element
It is not indentation you just did not add tab item for main screen, so it shows blank one for first view. So to fix this you have to add tab item:
TabView (selection: $selected){
mainscreen()
// << here !!
DayEntry().tabItem ({
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…