🐛 Fix incorrectly reported width when measuring lines

This commit fixes what seems to be a bug in Chromium.

When measuring lines (and with a special character sequence), it could happen
that Range(0, 0). getBoundingClientRect().width reports a number greater than 0.
This seems to happen when the font size is smaller than 12px and it's probably
due to subpixel font scaling. To solve it we've explicitly included a guard
clause that prevents this problem to happen.

spec/lines-yardstick-spec.coffee

@@ -2,7 +2,7 @@ LinesYardstick = require "../src/lines-yardstick"
 {toArray} = require 'underscore-plus'
 
 describe "LinesYardstick", ->
-  [editor, mockPresenter, mockLineNodesProvider, createdLineNodes, linesYardstick] = []
+  [editor, mockPresenter, mockLineNodesProvider, createdLineNodes, linesYardstick, buildLineNode] = []
 
   beforeEach ->
     waitsForPromise ->
@@ -49,10 +49,10 @@ describe "LinesYardstick", ->
           buildLineNode(screenRow)
         textNodesForLineIdAndScreenRow: (lineId, screenRow) ->
           lineNode = @lineNodeForLineIdAndScreenRow(lineId, screenRow)
+          iterator = document.createNodeIterator(lineNode, NodeFilter.SHOW_TEXT)
           textNodes = []
-          for span in lineNode.children
-            for textNode in span.childNodes
-              textNodes.push(textNode)
+          while textNode = iterator.nextNode()
+            textNodes.push(textNode)
           textNodes
 
       editor.setLineHeightInPixels(14)
@@ -126,6 +126,33 @@ describe "LinesYardstick", ->
       expect(linesYardstick.pixelPositionForScreenPosition([0, 9]).left).toBe 67
       expect(linesYardstick.pixelPositionForScreenPosition([0, 11]).left).toBe 84
 
+    it "doesn't report a width greater than 0 when the character to measure is at the beginning of a text node", ->
+      # This spec documents what seems to be a bug in Chromium, because we'd
+      # expect that Range(0, 0).getBoundingClientRect().width to always be zero.
+      atom.styles.addStyleSheet """
+      * {
+        font-size: 11px;
+        font-family: monospace;
+      }
+      """
+
+      text = "    \\vec{w}_j^r(\\text{new}) &= \\vec{w}_j^r(\\text{old}) + \\Delta\\vec{w}_j^r, \\\\"
+      buildLineNode = (screenRow) ->
+        lineNode = document.createElement("div")
+        lineNode.style.whiteSpace = "pre"
+        # We couldn't reproduce the problem with a simple string, so we're
+        # attaching the full one that comes from a bug report.
+        lineNode.innerHTML = '<span><span>  </span><span>  </span><span><span>\\</span>vec</span><span><span>{</span>w<span>}</span></span>_j^r(<span><span>\\</span>text</span><span><span>{</span>new<span>}</span></span>) &amp;= <span><span>\\</span>vec</span><span><span>{</span>w<span>}</span></span>_j^r(<span><span>\\</span>text</span><span><span>{</span>old<span>}</span></span>) + <span><span>\\</span>Delta</span><span><span>\\</span>vec</span><span><span>{</span>w<span>}</span></span>_j^r, <span>\\\\</span></span>'
+        jasmine.attachToDOM(lineNode)
+        createdLineNodes.push(lineNode)
+        lineNode
+
+      editor.setText(text)
+
+      expect(linesYardstick.pixelPositionForScreenPosition([0, 35]).left).toBe 230.90625
+      expect(linesYardstick.pixelPositionForScreenPosition([0, 36]).left).toBe 237.5
+      expect(linesYardstick.pixelPositionForScreenPosition([0, 37]).left).toBe 244.09375
+
     it "doesn't measure invisible lines if it is explicitly told so", ->
       atom.styles.addStyleSheet """
       * {

src/lines-yardstick.coffee

@@ -159,9 +159,12 @@ class LinesYardstick
       0
 
   leftPixelPositionForCharInTextNode: (lineNode, textNode, charIndex) ->
-    @rangeForMeasurement.setStart(textNode, 0)
-    @rangeForMeasurement.setEnd(textNode, charIndex)
-    width = @rangeForMeasurement.getBoundingClientRect().width
+    if charIndex is 0
+      width = 0
+    else
+      @rangeForMeasurement.setStart(textNode, 0)
+      @rangeForMeasurement.setEnd(textNode, charIndex)
+      width = @rangeForMeasurement.getBoundingClientRect().width
 
     @rangeForMeasurement.setStart(textNode, 0)
     @rangeForMeasurement.setEnd(textNode, textNode.textContent.length)