Stop avatars shifting the page during feature specs

spec/features/places/searching_a_place_spec.rb intermittently failed on
"goes to members' index page": the click on "View all members >>" landed
on nothing and the browser stayed put.

Member avatars come from gravatar.com over the network and were rendered
with img-fluid and no dimensions, so they reserved no space until they
arrived. The link sits directly under that grid. Capybara found the link,
an avatar landed, the link moved, and the click missed.

rails_helper meant to prevent this, but its guard tested for Poltergeist's
url_blacklist. Poltergeist is long gone and Selenium's browser has no such
method, so the condition was always false and the block never ran.

- Give the two avatar image tags explicit width and height, so the box is
  reserved before the image loads. This fixes real layout shift for
  members too, not only the specs.
- Replace the dead guard with config.x.blank_avatars, which browser specs
  set, making avatar_uri return a blank inline GIF. Helper specs still
  assert the real Gravatar URLs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/helpers/application_helper.rb

@@ -72,6 +72,12 @@ module ApplicationHelper
     text
   end
 
+  # A 1x1 transparent GIF, standing in for a real avatar when
+  # config.x.blank_avatars is set, which browser specs do: otherwise they fetch
+  # every avatar from gravatar.com for real, and one arriving mid-test moves
+  # whatever sits under it, so clicks land on the wrong thing.
+  BLANK_AVATAR = 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7'
+
   #
   # Returns an image uri for a given member.
   #
@@ -79,6 +85,7 @@ module ApplicationHelper
   #
   def avatar_uri(member, size = 150)
     return unless member
+    return BLANK_AVATAR if Rails.configuration.x.blank_avatars
 
     if member.preferred_avatar_uri.present?
       # Some avatars support different sizes

app/views/members/_avatar.html.haml

@@ -1 +1 @@
-= link_to image_tag(avatar_uri(member, 150), alt: "Avatar of #{member}", class: 'avatar img img-fluid'), member_path(member)
+= link_to image_tag(avatar_uri(member, 150), alt: "Avatar of #{member}", class: 'avatar img img-fluid', width: 150, height: 150), member_path(member)

app/views/members/_thumbnail.html.haml

@@ -5,7 +5,7 @@
     / Avatar
     .avatar.mx-auto.white
       = link_to member do
-        = image_tag(avatar_uri(member, 150), class: 'rounded-circle img-fluid', alt: member)
+        = image_tag(avatar_uri(member, 150), class: 'rounded-circle img-fluid', alt: member, width: 150, height: 150)
     .card-body
       / Name
       %h4.card-title

spec/rails_helper.rb

@@ -117,20 +117,22 @@ RSpec.configure do |config|
   # Allow just create(:factory) instead of needing to specify FactoryBot.create(:factory)
   config.include FactoryBot::Syntax::Methods
 
-  # Prevent Poltergeist from fetching external URLs during feature tests
+  config.after(:each, :js) do
+    Rails.configuration.x.blank_avatars = false
+  end
+
   config.before(:each, :js) do
     # TODO: Why are we setting this page size then straight afterwards, maximising?
     width = 1920
     height = 1080
     Capybara.current_session.driver.browser.manage.window.resize_to(width, height)
 
-    if page.driver.browser.respond_to?(:url_blacklist)
-      page.driver.browser.url_blacklist = [
-        'gravatar.com',
-        'okfn.org',
-        'googlecode.com'
-      ]
-    end
+    # This used to block gravatar.com and friends through Poltergeist's
+    # url_blacklist. Poltergeist is long gone and Selenium's browser has no
+    # such method, so the guard was always false and the block never ran —
+    # every browser spec has been fetching avatars from gravatar.com for real.
+    # Stand them down instead, so nothing loads late and shifts the page.
+    Rails.configuration.x.blank_avatars = true
 
     # Historically, we wanted to .maximize; but this actually undoes the resize_to step above
     # with chrome headless