🐛 Fix minimap size when splitting a pane

The original pane’s minimaps was still applying a margin
that was unnecessary.

lib/minimap-element.coffee

@@ -250,7 +250,7 @@ class MinimapElement extends HTMLElement
   update: ->
     return unless @attached and @isVisible() and not @minimap.isDestroyed()
 
-    if @adjustToSoftWrap
+    if @adjustToSoftWrap and @marginRight?
       @style.marginRight = @marginRight + 'px'
     else
       @style.marginRight = null
@@ -320,7 +320,11 @@ class MinimapElement extends HTMLElement
       softWrap = atom.config.get('editor.softWrap')
       width = lineLength * @minimap.getCharWidth()
 
-      @marginRight = width - @width if softWrap and lineLength and width < @width
+      if softWrap and lineLength and width < @width
+        @marginRight = width - @width
+      else
+        @marginRight = null
+
       canvasWidth = width
     else
       delete @marginRight

spec/minimap-element-spec.coffee

@@ -481,6 +481,19 @@ describe 'MinimapElement', ->
             nextAnimationFrame()
             expect(minimapElement.canvas.width).toEqual(4)
 
+      describe 'when the editor is resized', ->
+        beforeEach ->
+          atom.config.set 'editor.preferredLineLength', 6
+          editorElement.style.width = '100px'
+          editorElement.style.height = '100px'
+
+          sleep(150)
+          runs -> nextAnimationFrame()
+
+        it 'makes the minimap smaller than soft wrap', ->
+          expect(minimapElement.offsetWidth).toBeCloseTo(10, -1)
+          expect(minimapElement.style.marginRight).toEqual('')
+
       describe 'and when minimap.minimapScrollIndicator setting is true', ->
         beforeEach ->
           editor.setText(mediumSample)