browse-url-text-input-field is a variable defined in `browse-url.el'.
Its value is

Action on selecting an existing text browser buffer at an input field.
What to do when sending a new URL to an existing text browser buffer in Emacs
if the browser cursor is on an input field (in which case the `g' command
would be entered as data). Such fields are recognized by the
underlines ____. Allowed values: nil: disregard it, `warn': warn the
user and don't emit the URL, `avoid': try to avoid the field by moving
down (this *won't* always work).

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.