When a navigation starts, the URL bar does not immediately reflect the
loading URL in some cases (such as startup or certain initial loads). As a
result, the URL bar remains in a state that appears editable but is not yet
synchronized with the ongoing navigation.
If the user begins editing the URL during this period, the browser later
treats the navigation as the authoritative source of truth and overwrites the
URL bar when the load completes, discarding the user's edits.
By displaying the loading URL early, the URL bar is brought into a consistent
state with the navigation from the beginning. This ensures that subsequent
user edits are recognized as intentional overrides, and therefore are not
replaced when the load finishes.
Differential Revision: https://phabricator.services.mozilla.com/D291724
Daisuke Akatsuka
192265c24d