Decorate the stack frame lines within each mini-Editor

lib/stacktrace-view.coffee

@@ -81,11 +81,18 @@ class FrameView extends View
         grammar = atom.syntax.selectGrammar @frame.realPath, lines.join("\n")
         @source.getEditor().setGrammar grammar
 
-    @frame.getContext 3, (err, lines) =>
+    @frame.getContext 3, (err, lines, traceLine) =>
       if err?
         console.error err
       else
-        @source.getEditor().setText lines.join("\n")
+        editor = @source.getEditor()
+        editor.setText lines.join("\n")
+        range = editor.getBuffer().rangeForRow traceLine
+        @marker = editor.markBufferRange range
+        console.log editor.decorateMarker @marker, type: 'line', class: 'line-stackframe'
+
+  beforeRemove: ->
+    @marker.destroy() if @marker?
 
   navigate: ->
     @navCallback()

lib/stacktrace.coffee

@@ -144,9 +144,9 @@ class Frame
   # Public: Asynchronously collect n lines of context around the specified line number in this
   # frame's source file.
   #
-  # n        - The number of lines of context to collect on *each* side of the error line. The error
-  #            line will always be `lines[n]` and `lines.length` will be `n * 2 + 1`.
-  # callback - Invoked with any errors or an Array containing the relevant lines.
+  # n        - The number of lines of context to collect on *each* side of the error line.
+  # callback - Invoked with any errors, or an Array containing the relevant lines and the index of
+  #            the line in the Array that corresponds to the actual frame.
   #
   getContext: (n, callback) ->
     # Notice that @lineNumber is one-indexed, not zero-indexed.
@@ -155,7 +155,18 @@ class Frame
       toLine: @lineNumber + n
       trim: false
       keepLastEmptyLine: true
-    chomp fs.createReadStream(@realPath), range, callback
+    chomp fs.createReadStream(@realPath), range, (err, lines) =>
+      if err?
+        callback(err)
+      else
+        # Determine which line is the actual trace line.
+        if lines.length < (2 * n + 1) and range.fromLine < 0
+          # The front is cut off.
+          traceLine = @lineNumber - 1
+        else
+          traceLine = n
+
+        callback(null, lines, traceLine)
 
   navigateTo: ->
     position = [@lineNumber - 1, 0]

spec/stacktrace-spec.coffee

@@ -155,11 +155,12 @@ describe 'Frame', ->
     frame = new Frame('five', fixturePath, 5, 'something')
 
   it 'acquires n lines of context asynchronously', ->
-    lines = null
+    [lines, traceLine] = []
 
-    frame.getContext 2, (err, ls) ->
+    frame.getContext 2, (err, ls, lnum) ->
       throw err if err?
       lines = ls
+      traceLine = lnum
 
     waitsFor -> lines?
 
@@ -170,6 +171,7 @@ describe 'Frame', ->
       expect(lines[2]).toEqual('five')
       expect(lines[3]).toEqual('six')
       expect(lines[4]).toEqual('')
+      expect(traceLine).toBe(2)
 
   describe 'recognizes itself in an Editor', ->
     it 'is on a cursor', ->
@@ -180,3 +182,39 @@ describe 'Frame', ->
 
     it 'is on a different file', ->
       expect(frame.isOn(position: Point.fromObject([4, 0]), path: 'some/other/path.rb')).toBeFalsy()
+
+  it 'identifies the trace line if the beginning is cut off', ->
+    [lines, traceLine] = []
+    frame = new Frame('two', fixturePath, 2, 'something')
+
+    frame.getContext 3, (err, ls, lnum) ->
+      throw err if err?
+      lines = ls
+      traceLine = lnum
+
+    waitsFor -> lines?
+
+    runs ->
+      expect(lines.length).toBe(5)
+      expect(lines[0]).toEqual('one')
+      expect(lines[4]).toEqual('five')
+      expect(traceLine).toBe(1)
+      expect(lines[1]).toEqual('two')
+
+  it 'identifies the trace line if the end is cut off', ->
+    [lines, traceLine] = []
+    frame = new Frame('nine', fixturePath, 9, 'something')
+
+    frame.getContext 3, (err, ls, lnum) ->
+      throw err if err?
+      lines = ls
+      traceLine = lnum
+
+    waitsFor -> lines?
+
+    runs ->
+      expect(lines.length).toBe(6)
+      expect(lines[0]).toEqual('six')
+      expect(lines[5]).toEqual('')
+      expect(traceLine).toBe(3)
+      expect(lines[3]).toEqual('nine')

spec/stacktrace-view-spec.coffee

@@ -59,3 +59,6 @@ describe 'FrameView', ->
   it 'shows the function name', ->
     text = view.find('.function-name').text()
     expect(text).toEqual('midfunc')
+
+  it 'decorates the active line', ->
+    expect(view.find('.editor .line-stacktrace')).toHaveLength(1)