Additional context
I'd like to be able to do a layout like the one in the screenshot, as you can see the top "hero" content area (labeled Fluid container) fills the width of the page while the the next area (labelled Fixed container) has a defined width. I'd like to be able to achieve both easily in the same layout using Mosaic.
I still haven't dug deeply enough into "the Mosaic way" to know how feasible this is yet but the "end game" for me with Mosaic would involve being able to do something similar to systems like Mobirise.
Where you are able to have (1) preprepared block tiles (A block is a composite of a few types of tiles). (2) a block tile could be toggled between fluid and fixed.
See my mockup (a mashup of ideas from mobirise and mosaic). In the mockup there is a pull down sidebar on the left containing available block tiles. In the before panel one block tile is being dragged into the layout.
I did 'some trying' trying with this, and I found that the easiest was to set the the whole content area to 100%, and then add width to every tile that was not 'something'. It is actually possible to make a template that has some parts fluid and some not, but I felt that this ended up as a mess.
Slightly unrelated, but I am just making my first wordpress site, and I found an add on that does what you want. ( https://siteorigin.com ). This works pretty well, but I am not sure of 'how they do it'.
( the result is this, I assume there must be a javascript that calculates negative margins: http://egnevalg.no/wp/ )
They've put a lot of work into that page builder plugin and it does have many of the characteristics that I'm looking for. In fact it has additional things that I didn't think of but make sense. The video I included above is closer to the final goal though (ready made draggable combined blocks).
I am making a new theme, and to my big surprise I noticed that themingfragents now works as fluid.
This is great news, and means that I can put together pages like this: http://xweb14f.plana.dk:8080/systemtrafikk
by ONLY using mosaic.
This works basically OOTB. The only hack I did was to add a "clear floats" tile (basically just a div with a clear floats CSS class.
Quick note, mostly for my future self, that purge="False" is critical.
I missed it in my initial implementation and lots of the layout related menu disappeared!!
I think that we should look at a way to add support for containers (in the bootstrap world that would correspond to the container and container-fluid classes).
The hierarchy would be:
containers > rows > cells
Then rows could be added to the container and the container would be toggled to be either container-fluid or container.