TokenizedLine::screenColumnForBufferColumn calculates more accurately

screenColumnForBufferColumn used to break only if the current column
was strictly greater than the target column. This commit changes it so
it breaks when greater or equal, which is how bufferColumnForScreenColumn
works.

This also adds some unit tests for screenColumnForBufferColumn's
interactions with hard tab characters.

spec/display-buffer-spec.coffee

@@ -719,6 +719,14 @@ describe "DisplayBuffer", ->
       expect(displayBuffer.screenPositionForBufferPosition([100000, 0])).toEqual [12, 2]
       expect(displayBuffer.screenPositionForBufferPosition([100000, 100000])).toEqual [12, 2]
 
+    it "clips to the (left or right) edge of an atomic token without simply rounding up", ->
+      tabLength = 4
+      displayBuffer.setTabLength(tabLength)
+
+      buffer.insert([0, 0], '\t')
+      expect(displayBuffer.screenPositionForBufferPosition([0, 0])).toEqual [0, 0]
+      expect(displayBuffer.screenPositionForBufferPosition([0, 1])).toEqual [0, tabLength]
+
   describe "position translation in the presence of hard tabs", ->
     it "correctly translates positions on either side of a tab", ->
       buffer.setText('\t')

src/tokenized-line.coffee

@@ -67,7 +67,7 @@ class TokenizedLine
     screenColumn = 0
     currentBufferColumn = 0
     for token in @tokens
-      break if currentBufferColumn > bufferColumn
+      break if currentBufferColumn + token.bufferDelta > bufferColumn
       screenColumn += token.screenDelta
       currentBufferColumn += token.bufferDelta
     @clipScreenColumn(screenColumn + (bufferColumn - currentBufferColumn))