Make hack to break up adjacent spaces work for single-space tokens

Closes codemirror#4087

Author: marijnh

lib/codemirror.js

@@ -6944,6 +6944,7 @@
     var content = elt("span", null, null, webkit ? "padding-right: .1px" : null);
     var builder = {pre: elt("pre", [content], "CodeMirror-line"), content: content,
                    col: 0, pos: 0, cm: cm,
+                   trailingSpace: false,
                    splitSpaces: (ie || webkit) && cm.getOption("lineWrapping")};
     lineView.measure = {};
 
@@ -7005,7 +7006,7 @@
   // the line map. Takes care to render special characters separately.
   function buildToken(builder, text, style, startStyle, endStyle, title, css) {
     if (!text) return;
-    var displayText = builder.splitSpaces ? text.replace(/ {3,}/g, splitSpaces) : text;
+    var displayText = builder.splitSpaces ? splitSpaces(text, builder.trailingSpace) : text
     var special = builder.cm.state.specialChars, mustWrap = false;
     if (!special.test(text)) {
       builder.col += text.length;
@@ -7050,6 +7051,7 @@
         builder.pos++;
       }
     }
+    builder.trailingSpace = displayText.charCodeAt(text.length - 1) == 32
     if (style || startStyle || endStyle || mustWrap || css) {
       var fullStyle = style || "";
       if (startStyle) fullStyle += startStyle;
@@ -7061,11 +7063,17 @@
     builder.content.appendChild(content);
   }
 
-  function splitSpaces(old) {
-    var out = " ";
-    for (var i = 0; i < old.length - 2; ++i) out += i % 2 ? " " : "\u00a0";
-    out += " ";
-    return out;
+  function splitSpaces(text, trailingBefore) {
+    if (text.length > 1 && !/  /.test(text)) return text
+    var spaceBefore = trailingBefore, result = ""
+    for (var i = 0; i < text.length; i++) {
+      var ch = text.charAt(i)
+      if (ch == " " && spaceBefore && (i == text.length - 1 || text.charCodeAt(i + 1) == 32))
+        ch = "\u00a0"
+      result += ch
+      spaceBefore = ch == " "
+    }
+    return result
   }
 
   // Work around nonsense dimensions being reported for stretches of
@@ -7102,6 +7110,7 @@
       builder.content.appendChild(widget);
     }
     builder.pos += size;
+    builder.trailingSpace = false
   }
 
   // Outputs a number of spans to make up a line, taking highlighting