- Set every element of a form and every sub-group (field+label) to „Resize: None“.
- Place the „Send“ button and its message field centred.
- pin every sub-group, the „Send“ button and the „Send“ message centred.
- Don’t believe, what you see, when you drag the scrubber, but preview in browser.
I found the error of my ways. I missed a few "sub-groups" (i.e., text field connected to field descriptor). Went back and checked all, pinned everything center (as you instructed!), and all is well now. Many, many thanks!
Nevertheless: It is really laborious to use a centred, fixed width form on a responsive page!