I'd still like to know whether the QFormLayout vertical spacing is "unavoidable" because it varies per style at runtime or whether it should really be shown "squeezed" at design time because all styles do so anyway....

Btw, is there a way in Designer to preview the final look at runtime without having to run the program? I kinda thought I remembered an Alt-D but that does not seem to do so, maybe mixing up with another IDE.

Tools -> Form Editor -> Preview... (alt-shift-r on macOS)
The Preview In submenu below it offers previews in a few styles.

• Either the same one as you designing/running on (I never set the style, so it's always platform default); and
• Looking at the 4 dialog style pictures in https://doc.qt.io/qt-6/qformlayout.html#details and assuming they are runtime pix, although I admit it's hard to tell none of them show any evidence of "vertical stretching" of the rows. Unless that is the case on some platforms (is it actually?), the obvious answer is to show "squeezed" rather than "spaced" at design time too....

I know you are not a great fan of UI design. I am not too, and I won't die over it. But when one does use it one wants it to be as close as possible to runtime, unless there is a good reason not to be.

Btw, is there a way in Designer to preview the final look at runtime without having to run the program? I kinda thought I remembered an Alt-D but that does not seem to do so, maybe mixing up with another IDE.

It knows it's a QFormLayout just as much when designing as running.

What guidline should it follow during design? https://doc.qt.io/qt-6/qformlayout.html#details

It's a feature of QFormLayout - it creates a properly aligned two-column layout and aligns them on top

I'm sure it is, that was not the question. The question is: given that, why does it not show that at design time (it shows the horizontal positioning but not correctly the vertical positioning), why only at runtime? It knows it's a QFormLayout just as much when designing as running.

If you are going to have a visual editor, part of the whole point is that it shows at design as it will at run. I know there are difficult or edge or even platform-specific cases where that might not possible, but what makes it so here? I don't think this is a trivial point/question, could you give an indication as to why this case does not work like that? Is this specific to QFormLayout, it makes me have no trust that e.g. any layout will look the same when it seems easy to make it so?

It's a feature of QFormLayout - it creates a properly aligned two-column layout and aligns them on top