Update foldability indicators on changes that don't insert/remove lines

On changes that insert/remove lines, we redraw the entire gutter, which
ensures the foldability markers are up to date. But other kinds of
changes could change the foldability of lines… if a line becomes
commented or its indentation changes, the foldability of that line and
the lines above and below could be affected. For now, the gutter
understands this fact and updates the foldability for the region
extending from one line before to one line after each change. Someday
I'd like the view logic to not bake in knowledge of the folding rules,
but this works for now.

spec/editor-view-spec.coffee

@@ -1829,6 +1829,21 @@ describe "EditorView", ->
       expect(editorView.gutter.find('.line-number:eq(3)')).not.toHaveClass 'foldable'
       expect(editorView.gutter.find('.line-number:eq(4)')).toHaveClass 'foldable'
 
+      # changes to indentation update foldability
+      editor.setIndentationForBufferRow(1, 0)
+      expect(editorView.gutter.find('.line-number:eq(0)')).not.toHaveClass 'foldable'
+      expect(editorView.gutter.find('.line-number:eq(1)')).toHaveClass 'foldable'
+
+      # changes to comments update foldability
+      editor.toggleLineCommentsForBufferRows(2, 3)
+      expect(editorView.gutter.find('.line-number:eq(2)')).toHaveClass 'foldable'
+      expect(editorView.gutter.find('.line-number:eq(3)')).not.toHaveClass 'foldable'
+      editor.toggleLineCommentForBufferRow(2)
+      expect(editorView.gutter.find('.line-number:eq(2)')).not.toHaveClass 'foldable'
+      expect(editorView.gutter.find('.line-number:eq(3)')).not.toHaveClass 'foldable'
+      editor.toggleLineCommentForBufferRow(4)
+      expect(editorView.gutter.find('.line-number:eq(3)')).toHaveClass 'foldable'
+
     describe "when lines are inserted", ->
       it "re-renders the correct line number range in the gutter", ->
         editorView.scrollTop(3 * editorView.lineHeight)

src/gutter.coffee

@@ -151,10 +151,12 @@ class Gutter extends View
           updateAllLines = true
           break
 
+    # If a change added or removed lines, just rebuild the entire gutter
     if updateAllLines
       @lineNumbers[0].innerHTML = @buildLineElementsHtml(startScreenRow, endScreenRow)
+
+    # Handle changes that didn't add/remove lines more optimally
     else
-      # When scrolling or adding/removing lines, we just add/remove lines from the ends.
       if startScreenRow < @firstScreenRow
         @prependLineElements(@buildLineElements(startScreenRow, @firstScreenRow-1))
       else if startScreenRow != @firstScreenRow
@@ -165,6 +167,8 @@ class Gutter extends View
       else if endScreenRow != @lastScreenRow
         @removeLineElements(endScreenRow - @lastScreenRow)
 
+      @updateFoldableClasses(changes)
+
     @firstScreenRow = startScreenRow
     @lastScreenRow = endScreenRow
     @highlightedRows = null
@@ -219,6 +223,23 @@ class Gutter extends View
 
     html
 
+  # Private: Called to update the 'foldable' class of line numbers when there's
+  # a change to the display buffer that doesn't regenerate all the line numbers
+  # anyway.
+  updateFoldableClasses: (changes) ->
+    editor = @getEditor()
+    for {start, end} in changes when start <= @lastScreenRow
+      startScreenRow = Math.max(start - 1, @firstScreenRow)
+      endScreenRow =  Math.min(end + 1, @lastScreenRow)
+      lastBufferRow = null
+      for bufferRow in editor.bufferRowsForScreenRows(startScreenRow, endScreenRow) when bufferRow isnt lastBufferRow
+        lastBufferRow = bufferRow
+        lineNumberElement = @getLineNumberElement(bufferRow)[0]
+        if editor.isFoldableAtBufferRow(bufferRow)
+          lineNumberElement.classList.add('foldable')
+        else
+          lineNumberElement.classList.remove('foldable')
+
   removeLineHighlights: ->
     return unless @highlightedLineNumbers
     for line in @highlightedLineNumbers