Fix textStyles commands

src/WpfMath.Tests/BoxTests.fs

@@ -115,3 +115,11 @@ let wideItemInMatrixBox() =
 [<Fact>]
 let emptyColorbox(): unit =
     verifyBox @"\colorbox{red}{}"
+
+[<Fact>]
+let emptyMathrm(): unit =
+    verifyBox @"\mathrm{}"
+
+[<Fact>]
+let emptyCommandText(): unit =
+    verifyBox @"\text{}"

src/WpfMath.Tests/TestResults/BoxTests.emptyCommandText.approved.txt

@@ -0,0 +1,18 @@
+{
+  "Children": [],
+  "Source": {
+    "Start": 1,
+    "End": 7,
+    "Length": 6,
+    "Source": "\\text{}"
+  },
+  "Foreground": null,
+  "Background": null,
+  "TotalHeight": 0.0,
+  "TotalWidth": 0.0,
+  "Italic": 0.0,
+  "Width": 0.0,
+  "Height": 0.0,
+  "Depth": 0.0,
+  "Shift": 0.0
+}

src/WpfMath.Tests/TestResults/BoxTests.emptyMathrm.approved.txt

@@ -0,0 +1,18 @@
+{
+  "Children": [],
+  "Source": {
+    "Start": 1,
+    "End": 9,
+    "Length": 8,
+    "Source": "\\mathrm{}"
+  },
+  "Foreground": null,
+  "Background": null,
+  "TotalHeight": 0.0,
+  "TotalWidth": 0.0,
+  "Italic": 0.0,
+  "Width": 0.0,
+  "Height": 0.0,
+  "Depth": 0.0,
+  "Shift": 0.0
+}

src/WpfMath/TexFormulaParser.cs

@@ -572,13 +572,15 @@ private void ProcessEscapeSequence(TexFormula formula,
                 // Text style was found.
                 SkipWhiteSpace(value, ref position);
 
-                var styledFormula = command == TexUtilities.TextStyleName
-                    ? ConvertRawText(ReadElement(value, ref position), command)
-                    : Parse(ReadElement(value, ref position), command, environment.CreateChildEnvironment());
-
-                var atom = AttachScripts(formula, value, ref position, styledFormula.RootAtom, true, environment);
-                var source = new SourceSpan(formulaSource.Source, formulaSource.Start, position);
-                formula.Add(atom, source);
+                var remainingString = ReadElement(value, ref position);
+                var remaining = command == TexUtilities.TextStyleName
+                    ? ConvertRawText(remainingString, command)
+                    : Parse(remainingString, command, environment.CreateChildEnvironment());
+
+                var source = value.Segment(start, position - start);
+                var styledAtom = new StyledAtom(source, remaining.RootAtom, null, null);
+                var commandAtom = AttachScripts(formula, value, ref position, styledAtom, true, environment);
+                formula.Add(commandAtom, source);
             }
             else if (embeddedCommands.Contains(command)
                  || environment.AvailableCommands.ContainsKey(command)