🐛 Fix minimap redraw when scrolling down

By adding an extra line to the canvas we are sure to render more than
the height and have enough content to paste on the offscreen buffer

lib/minimap-element.coffee

@@ -41,13 +41,12 @@ class MinimapElement extends HTMLElement
     @shadowRoot.appendChild(@visibleArea)
 
   measureHeightAndWidth: ->
-    editorElement = @getTextEditorElement()
-    width = @clientWidth
-    height = @clientHeight
+    @width = @clientWidth
+    @height = @clientHeight
 
-    if width isnt @canvas.width or height isnt @canvas.height
-      @canvas.width = width * devicePixelRatio
-      @canvas.height = height * devicePixelRatio
+    if @width isnt @canvas.width or @height isnt @canvas.height
+      @canvas.width = @width * devicePixelRatio
+      @canvas.height = (@height + @minimap.getLineHeight()) * devicePixelRatio
 
   getTextEditor: -> @minimap.getTextEditor()
 

spec/minimap-element-spec.coffee

@@ -93,7 +93,7 @@ describe 'MinimapElement', ->
       expect(minimapElement.offsetWidth).toBeCloseTo(editorElement.clientWidth / 10, -1)
 
     it 'resizes the canvas to fit the minimap', ->
-      expect(canvas.offsetHeight).toEqual(minimapElement.offsetHeight)
+      expect(canvas.offsetHeight).toEqual(minimapElement.offsetHeight + minimap.getLineHeight())
       expect(canvas.offsetWidth).toEqual(minimapElement.offsetWidth)
 
     it 'requests an update', ->