I often use things like flexbox to order data (instead of layout grids).
I wish there was a layout grid that didnt create "layout grid dom" so that I could visualize the data in the modeler the same way it would render onscreen.
The way I fix it now is to add text to the containers and then with css hide the "labels" of the containers.
This way the dev's know where to put the content, and I can still use fancy flexbox.
To be honest the solution for this is rather complex if you want to make it reflect your CSS classes and defintions, because right now grid and flex box are popular concepts for structuring pages. And these standards seem to be ever evolving because of performance of each new concept introduced.
But also just adding classes like inline-block, float: left; float: right; clear: left; clear: right; clear:both;
All these concepts would require to be displayed properly within the Mendix platform.
And personally I would like to keep the low fidelity visuals in the Mendix modeler as it allows for better conceptual thinking without focus on the visuals right away.
I reckon there should come a solution, just wanted to say that there is a lot more to it than just supporting flex box.
Perhaps a quick solution would be some alignment tools for each widget in the page. And a toggle to display it inline with another element. But this would also require each element to get "width and height" attributes independent of what is defined in the CSS as it should be scaled against the view port width and height for displaying in the modeler.
I have seen it, It's a fun feature but doesnt seem to serve our team with any benefits.
It also doesnt seem to render correctly, so has gone unused.
The problem is that the view mode doesnt allow you to edit content intuitively (and also feels untrustworthy due to it rendering differently then the browser, also modal popups render differently, and seeing as the browser is already open, it just gives you a worse version of something you already have access too (with correct data)).
Have you taken a look at Mendix 7.16.0 with the view mode of pages inside the Modeler? You cannot edit the page in that view, but perhaps it can help your team?