Fix a serious bug in midpoint placement

This only compared using `==` which means that we misidentify a lot of
open-ranges when calculating `move_to_end`/`move_to_start`.

app/models/concerns/relative_positioning.rb

@@ -83,9 +83,9 @@ module RelativePositioning
       if gap_width < MIN_GAP
         raise NoSpaceLeft
       elsif gap_width > MAX_GAP
-        if pos_before == MIN_POSITION
+        if pos_before <= MIN_POSITION
           pos_after - IDEAL_DISTANCE
-        elsif pos_after == MAX_POSITION
+        elsif pos_after >= MAX_POSITION
           pos_before + IDEAL_DISTANCE
         else
           midpoint

spec/support/shared_examples/models/relative_positioning_shared_examples.rb

@@ -568,6 +568,12 @@ RSpec.shared_examples 'a class that supports relative positioning' do
       end
     end
 
+    it 'places items at most IDEAL_DISTANCE from the start when the range is open' do
+      n = item1.send(:scoped_items).count
+
+      expect([item1, item2].map(&:relative_position)).to all(be >= (RelativePositioning::START_POSITION - (n * RelativePositioning::IDEAL_DISTANCE)))
+    end
+
     it 'moves item to the end' do
       new_item.move_to_start
 
@@ -613,6 +619,12 @@ RSpec.shared_examples 'a class that supports relative positioning' do
       end
     end
 
+    it 'places items at most IDEAL_DISTANCE from the start when the range is open' do
+      n = item1.send(:scoped_items).count
+
+      expect([item1, item2].map(&:relative_position)).to all(be <= (RelativePositioning::START_POSITION + (n * RelativePositioning::IDEAL_DISTANCE)))
+    end
+
     it 'moves item to the end' do
       new_item.move_to_end