MDL-48865 core_block: Show regions with only hidden blocks when editing

Before this patch if all the blocks in a block region are set to hidden the region will be docked even when editing is enabled.
This meant that as a user I would need to add a new block to the page and move it to that region via the configuration
menu before I could manipulate the hidden blocks again.

blocks/tests/behat/hidden_block_region.feature

@@ -0,0 +1,49 @@
+@core @core_block
+Feature: Show hidden blocks in a docked block region when editing
+  In order to edit blocks in a hidden region
+  As a teacher
+  I need to be able to see the blocks when editing is on
+
+  Background:
+    Given the following "courses" exist:
+      | fullname | shortname | format |
+      | Course 1 | C1 | topics |
+    And the following "course enrolments" exist:
+      | user | course | role |
+      | admin | C1 | editingteacher |
+    And I log in as "admin"
+    And I follow "Course 1"
+    And I turn editing mode on
+    # Hide all the blocks in the non-default region
+    And I configure the "Search forums" block
+    And I set the following fields to these values:
+      | Visible | No |
+    And I click on "Save changes" "button"
+    And I configure the "Latest news" block
+    And I set the following fields to these values:
+      | Visible | No |
+    And I click on "Save changes" "button"
+    And I configure the "Upcoming events" block
+    And I set the following fields to these values:
+      | Visible | No |
+    And I click on "Save changes" "button"
+    And I configure the "Recent activity" block
+    And I set the following fields to these values:
+      | Visible | No |
+    When I click on "Save changes" "button"
+    # Editing is on so they should be visible
+    Then I should see "Search forums"
+    And I should see "Latest news"
+    And I should see "Upcoming events"
+    And I should see "Recent activity"
+    And I turn editing mode off
+    # Editing is off, so they should no longer be visible
+    And I should not see "Search forums"
+    And I should not see "Latest news"
+    And I should not see "Upcoming events"
+    And I should not see "Recent activity"
+
+  @javascript
+  Scenario: Check that a region with only hidden blocks is not docked in editing mode (javascript enabled)
+
+  Scenario: Check that a region with only hidden blocks is not docked in editing mode (javascript disabled)

lib/blocklib.php

@@ -486,6 +486,11 @@ public function region_completely_docked($region, $output) {
             return false;
         }
 
+        // Block regions should not be docked during editing when all the blocks are hidden.
+        if ($this->page->user_is_editing() && $this->page->user_can_edit_blocks()) {
+            return false;
+        }
+
         $this->check_is_loaded();
         $this->ensure_content_created($region, $output);
         if (!$this->region_has_content($region, $output)) {