Add a dummy scrollbar corner

Horizontal / vertical scrollbars render a 'corner' on the lower right
when they would otherwise overlap. I previously relied on drawing both
dummy scrollbars at their full width/height so the corner got rendered,
but that interfered with the display of the horizontal scrollbar in
certain circumstances because it was too wide to scroll. This commit
provides that behavior with an absolutely positioned div with the same
dimensions as the intersection of scrollbars when both are visible.

src/editor-component.coffee

@@ -5,6 +5,7 @@ React = require 'react'
 GutterComponent = require './gutter-component'
 EditorScrollViewComponent = require './editor-scroll-view-component'
 ScrollbarComponent = require './scrollbar-component'
+ScrollbarCornerComponent = require './scrollbar-corner-component'
 SubscriberMixin = require './subscriber-mixin'
 
 module.exports =
@@ -36,6 +37,8 @@ EditorComponent = React.createClass
       lineHeightInPixels = editor.getLineHeight()
       horizontalScrollbarHeight = editor.getHorizontalScrollbarHeight()
       verticalScrollbarWidth = editor.getVerticalScrollbarWidth()
+      verticallyScrollable = editor.verticallyScrollable()
+      horizontallyScrollable = editor.horizontallyScrollable()
 
     className = 'editor editor-colors react'
     className += ' is-focused' if focused
@@ -61,7 +64,7 @@ EditorComponent = React.createClass
         onScroll: @onVerticalScroll
         scrollTop: scrollTop
         scrollHeight: scrollHeight
-        scrollableInOppositeDirection: editor.horizontallyScrollable() if @isMounted()
+        scrollableInOppositeDirection: horizontallyScrollable
         horizontalScrollbarHeight: horizontalScrollbarHeight
 
       ScrollbarComponent
@@ -71,9 +74,14 @@ EditorComponent = React.createClass
         onScroll: @onHorizontalScroll
         scrollLeft: scrollLeft
         scrollWidth: scrollWidth + @gutterWidth
-        scrollableInOppositeDirection: editor.verticallyScrollable() if @isMounted()
+        scrollableInOppositeDirection: verticallyScrollable
         verticalScrollbarWidth: verticalScrollbarWidth
 
+      ScrollbarCornerComponent
+        visible: horizontallyScrollable and verticallyScrollable
+        height: horizontalScrollbarHeight
+        width: verticalScrollbarWidth
+
   getRenderedRowRange: ->
     renderedRowRange = @props.editor.getVisibleRowRange()
     if @preservedRowRange?

src/scrollbar-corner-component.coffee

@@ -0,0 +1,13 @@
+React = require 'react'
+{div} = require 'reactionary'
+
+module.exports =
+ScrollbarComponent = React.createClass
+  render: ->
+    {visible, width, height} = @props
+    display = 'none' unless visible
+
+    div className: 'scrollbar-corner', style: {display, width, height},
+      div style:
+        height: height + 1
+        width: width + 1