When opening different dialogs, specially those that are url-based (iframe), the content wiggles around producing a sensation of instability.
This is caused by different classes being added to the iframe content after it's fully loaded, causing different css to force several layouts and reflows.
By providing a more consistent experience, avoiding the content to move around while visible, we would provide a more pleasant experience.
This is in fact a follow-up of
LPS-94229 for inner navigation where some flickering still occurs.