When one grasps where to use Flexible Blocks and where to use Freehand Blocks, design gets easier:

Here's today's update: included the use of Clip View to create an interesting effect that stretches the full browser width, and a full browser width slider (the latter being in use on several traditional sites):

https://initiostar.co.uk/demo/blocks2/

Sadly, I can still replicate chaos by placing an object or group of objects, where the horizontal dimension of the object or group of objects is bigger that those of the FlexGroup within the FlexBlock. The FlexGroup extends the FlexBlock and one has a lot of fiddling about to reset it.

@ Chris, the only block I could see in your example with its width extended beyond the page width was the widget: I went to the P&L Gallery, SOLO select the FlexGroup, dragged all contents out the block, resized the FlexGroup, BlockBackground and Block; made sure the Widget size was reduced to fit the within the FlexGroup, added it back; all is well (though no content as it was not connected).

FlexiThing_Widget_Alighment_1.xar

I did get a message (copied into the above file) that the number of matching Web Blocks in variants were different, but that might be historic.