Fix tab causing text overflow bug.

Runtime/ui/txt/layout.cs

             float letterSpaceHalfRight = letterSpace - letterSpaceHalfLeft;
 
             for (int i = 0; i < wordCount; i++) {
+                initAdvance = x;
-                    if (advances != null) {
-                        advances[i + layoutOffset] = letterSpaceHalfLeft + wordSpacing;
-                    }
+                    advances[i + layoutOffset] = letterSpaceHalfLeft + wordSpacing;
-                    if (advances != null) {
-                        advances[i - 1 + layoutOffset] += letterSpaceHalfRight;
-                        advances[i + layoutOffset] = letterSpaceHalfLeft;
-                    }
+                    advances[i - 1 + layoutOffset] += letterSpaceHalfRight;
+                    advances[i + layoutOffset] = letterSpaceHalfLeft;
 
                     x += letterSpace;
                 }
                 }
 
-                if (positions != null) {
-                    positions[i + layoutOffset] = x;
-                }
+                positions[i + layoutOffset] = x;
 
                 float advance;
                 if (ch == '\t') {
                 }
 
                 x += advance;
-                if (advances != null) {
-                    advances[i + layoutOffset] += advance;
-                }
+                advances[i + layoutOffset] += advance;
-                    if (advances != null) {
-                        advances[i + layoutOffset] += letterSpaceHalfRight;
-                    }
-
+                    advances[i + layoutOffset] += letterSpaceHalfRight;
                     x += letterSpaceHalfRight;
                 }
             }
                     float letterSpaceHalfRight = letterSpace - letterSpaceHalfLeft;
 
                     x += letterSpaceHalfLeft;
-                    if (advances != null) {
-                        advances[i] = letterSpaceHalfLeft;
-                    }
+                    advances[i] = letterSpaceHalfLeft;
 
 
                     var minX = x;
                     _updateBounds(minX, maxX, minY, maxY, ref bounds);
 
-                    if (positions != null) {
-                        positions[i] = x;
-                    }
+                    positions[i] = x;
-                    if (advances != null) {
-                        advances[i] += advance;
-                        advances[i] += letterSpaceHalfRight;
-                    }
+                    advances[i] += advance;
+                    advances[i] += letterSpaceHalfRight;
-                    if (advances != null) {
-                        advances[i] = 0;
-                    }
-
-                    if (positions != null) {
-                        positions[i] = x;
-                    }
+                    advances[i] = 0;
+                    positions[i] = x;
                 }
             }
 

Runtime/ui/txt/paragraph.cs

 
             var offset = position.offset;
             if (position.affinity == TextAffinity.upstream && offset > 0) {
-                offset = _isUtf16Surrogate(this._text[offset - 1]) ? offset - 2 : offset - 1;
+                offset = char.IsSurrogate(this._text[offset - 1]) ? offset - 2 : offset - 1;
-            for (int lineIndex = 0; lineIndex < this.getLineCount(); ++lineIndex) {
+            for (int lineIndex = 0; lineIndex < lineCount; ++lineIndex) {
                 var line = this._lineRanges[lineIndex];
                 if ((offset >= line.start && offset < line.endIncludingNewLine)) {
                     return lineIndex;
             }
 
             return 0;
-        }
-
-        static bool _isUtf16Surrogate(int value) {
-            return (value & 0xF800) == 0xD800;
         }
     }