Allow to disable the Performance Bar and document the `p b` shortcut in its doc page

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/models/application_setting.rb

@@ -420,7 +420,7 @@ class ApplicationSetting < ActiveRecord::Base
   #   the enabling/disabling is `performance_bar_allowed_group_id`
   # - If `enable` is false, we set `performance_bar_allowed_group_id` to `nil`
   def performance_bar_enabled=(enable)
-    return if enable
+    return if Gitlab::Utils.to_boolean(enable)
     self.performance_bar_allowed_group_id = nil
   end

changelogs/unreleased/39570-performance-bar-appears-enabled-even-though-it-won-t-show-up.yml

+---
+title: Allow to disable the Performance Bar
+merge_request: 15084
+author:
+type: fixed

doc/administration/monitoring/performance/performance_bar.md

@@ -28,6 +28,12 @@ will be allowed to display the Performance Bar.
 Make sure _Enable the Performance Bar_ is checked and hit
 **Save** to save the changes.
+Once the Performance Bar is enabled, you will need to press the [<kbd>p</kbd> +
+<kbd>b</kbd> keyboard shortcut](../../../workflow/shortcuts.md) to actually
+display it.
+
+You can toggle the Bar using the same shortcut.
+
 ---
 ![GitLab Performance Bar Admin Settings](img/performance_bar_configuration_settings.png)

doc/workflow/shortcuts.md

@@ -9,7 +9,7 @@ You can see GitLab's keyboard shortcuts by using 'shift + ?'
 | <kbd>n</kbd> | Main navigation |
 | <kbd>s</kbd> | Focus search |
 | <kbd>f</kbd> | Focus filter |
-| <kbd>p b</kbd> | Show/hide the Performance Bar |
+| <kbd>p</kbd> + <kbd>b</kbd> | Show/hide the Performance Bar |
 | <kbd>?</kbd> | Show/hide this dialog |
 | <kbd>⌘</kbd> + <kbd>shift</kbd> + <kbd>p</kbd> | Toggle markdown preview |
 | <kbd>↑</kbd> | Edit last comment (when focused on an empty textarea) |

spec/features/admin/admin_settings_spec.rb

@@ -95,6 +95,29 @@ feature 'Admin updates settings' do
     expect(find_field('ED25519 SSH keys').value).to eq(forbidden)
   end
+  scenario 'Change Performance Bar settings' do
+    group = create(:group)
+
+    check 'Enable the Performance Bar'
+    fill_in 'Allowed group', with: group.path
+
+    click_on 'Save'
+
+    expect(page).to have_content 'Application settings saved successfully'
+
+    expect(find_field('Enable the Performance Bar')).to be_checked
+    expect(find_field('Allowed group').value).to eq group.path
+
+    uncheck 'Enable the Performance Bar'
+
+    click_on 'Save'
+
+    expect(page).to have_content 'Application settings saved successfully'
+
+    expect(find_field('Enable the Performance Bar')).not_to be_checked
+    expect(find_field('Allowed group').value).to be_nil
+  end
+
   def check_all_events
     page.check('Active')
     page.check('Push')