[4.0] Don't move elements on page when a span appears (Joomla Installer)

[simbus82]

Steps to reproduce the issue

Start to install a J4 Nightly
Focus on some elements

Expected result

No elements shift (wrong for UX)

Actual result

A span is pushing elements away

Additional comments

To obtain the expected result I have added a float:right (and a margin) to the span that contain the "Please fill in this field".
I don't think it's the correct thing to do. There is an upstream problem related to the fact that "flexbox" has not been used and therefore everything is positioned relatively.