Handle moving to start/end with one gap left

When moving to the start / end, the min / max position must also be
available to position the new item to

app/models/concerns/relative_positioning.rb

@@ -297,13 +297,13 @@ module RelativePositioning
 
     if max_pos.nil?
       self.relative_position = START_POSITION
-    elsif gap_too_small?(max_pos, MAX_POSITION)
+    elsif gap_too_small?(max_pos, MAX_POSITION + 1)
       max = relative_siblings.order(Gitlab::Database.nulls_last_order('relative_position', 'DESC')).first
       max.move_sequence_before(true)
       max.reset
-      self.relative_position = self.class.position_between(max.relative_position, MAX_POSITION)
+      self.relative_position = self.class.position_between(max.relative_position, MAX_POSITION + 1)
     else
-      self.relative_position = self.class.position_between(max_pos, MAX_POSITION)
+      self.relative_position = self.class.position_between(max_pos, MAX_POSITION + 1)
     end
   end
 
@@ -312,13 +312,13 @@ module RelativePositioning
 
     if min_pos.nil?
       self.relative_position = START_POSITION
-    elsif gap_too_small?(min_pos, MIN_POSITION)
+    elsif gap_too_small?(min_pos, MIN_POSITION - 1)
       min = relative_siblings.order(Gitlab::Database.nulls_last_order('relative_position', 'ASC')).first
       min.move_sequence_after(true)
       min.reset
-      self.relative_position = self.class.position_between(MIN_POSITION, min.relative_position)
+      self.relative_position = self.class.position_between(MIN_POSITION - 1, min.relative_position)
     else
-      self.relative_position = self.class.position_between(MIN_POSITION, min_pos)
+      self.relative_position = self.class.position_between(MIN_POSITION - 1, min_pos)
     end
   end
 

spec/support/shared_examples/models/relative_positioning_shared_examples.rb

@@ -512,6 +512,14 @@ RSpec.shared_examples 'a class that supports relative positioning' do
       expect(new_item.relative_position).to be < item2.relative_position
     end
 
+    it 'positions the item at MIN_POSITION when there is only one space left' do
+      item2.update!(relative_position: RelativePositioning::MIN_POSITION + 1)
+
+      new_item.move_to_start
+
+      expect(new_item.relative_position).to eq RelativePositioning::MIN_POSITION
+    end
+
     it 'rebalances when there is already an item at the MIN_POSITION' do
       item2.update!(relative_position: RelativePositioning::MIN_POSITION)
 
@@ -549,6 +557,14 @@ RSpec.shared_examples 'a class that supports relative positioning' do
       expect(new_item.relative_position).to be > item2.relative_position
     end
 
+    it 'positions the item at MAX_POSITION when there is only one space left' do
+      item2.update!(relative_position: RelativePositioning::MAX_POSITION - 1)
+
+      new_item.move_to_end
+
+      expect(new_item.relative_position).to eq RelativePositioning::MAX_POSITION
+    end
+
     it 'rebalances when there is already an item at the MAX_POSITION' do
       item2.update!(relative_position: RelativePositioning::MAX_POSITION)