Use MSWord compatible RTF sequence for background text color (#16035)

The `GenRTF(...)` was using `\highlight` control word for sending
background text color in the RTF format during a copy command. This
doesn't work correctly, since many applications (E.g. MSWord) don't
support full RGB with `\highlight`, and instead uses an approximation of
what is received. For example, `rgb(197, 15, 31)` becomes `rgb(255, 0,
255)`. Also, the standard way of using background colors is `\cbN`
control word, which isn't supported as per the [RTF Spec 1.9.1]
in Word.

But it briefly mentioned a workaround at Pg. 23, which seems to work on
all the RTF editors I tested.

The PR makes the changes to use `\chshdng0\chcbpatN` for the background
coloring.

Also did some refactoring to make the implementation concise.

## Validation Steps Performed

Verified that the background is correctly copied on below editors:
- MSWord
- WordPad
- LibreOffice
- Outlook

[RTF Spec 1.9.1]: https://msopenspecs.azureedge.net/files/Archive_References/[MSFT-RTF].pdf

.github/actions/spelling/expect/expect.txt

@@ -188,8 +188,10 @@ changelist
 chaof
 charinfo
 CHARSETINFO
+chcbpat
 chh
 chk
+chshdng
 CHT
 Cic
 cielab

src/buffer/out/textBuffer.cpp

@@ -2195,6 +2195,7 @@ std::string TextBuffer::GenHTML(const TextAndColor& rows,
 // Routine Description:
 // - Generates an RTF document based on the passed in text and color data
 //   RTF 1.5 Spec: https://www.biblioscape.com/rtf15_spec.htm
+//   RTF 1.9.1 Spec: https://msopenspecs.azureedge.net/files/Archive_References/[MSFT-RTF].pdf
 // Arguments:
 // - rows - the text and color data we will format & encapsulate
 // - backgroundColor - default background color for characters, also used in padding
@@ -2214,10 +2215,18 @@ std::string TextBuffer::GenRTF(const TextAndColor& rows, const int fontHeightPoi
 
         // Standard RTF header.
         // This is similar to the header generated by WordPad.
-        // \ansi - specifies that the ANSI char set is used in the current doc
-        // \ansicpg1252 - represents the ANSI code page which is used to perform the Unicode to ANSI conversion when writing RTF text
-        // \deff0 - specifies that the default font for the document is the one at index 0 in the font table
-        // \nouicompat - ?
+        // \ansi:
+        //   Specifies that the ANSI char set is used in the current doc.
+        // \ansicpg1252:
+        //   Represents the ANSI code page which is used to perform
+        //   the Unicode to ANSI conversion when writing RTF text.
+        // \deff0:
+        //   Specifies that the default font for the document is the one
+        //   at index 0 in the font table.
+        // \nouicompat:
+        //   Some features are blocked by default to maintain compatibility
+        //   with older programs (Eg. Word 97-2003). `nouicompat` disables this
+        //   behavior, and unblocks these features. See: Spec 1.9.1, Pg. 51.
         rtfBuilder << "\\rtf1\\ansi\\ansicpg1252\\deff0\\nouicompat";
 
         // font table
@@ -2226,17 +2235,25 @@ std::string TextBuffer::GenRTF(const TextAndColor& rows, const int fontHeightPoi
         // map to keep track of colors:
         // keys are colors represented by COLORREF
         // values are indices of the corresponding colors in the color table
-        std::unordered_map<COLORREF, int> colorMap;
-        auto nextColorIndex = 1; // leave 0 for the default color and start from 1.
+        std::unordered_map<COLORREF, size_t> colorMap;
 
         // RTF color table
         std::ostringstream colorTableBuilder;
         colorTableBuilder << "{\\colortbl ;";
-        colorTableBuilder << "\\red" << static_cast<int>(GetRValue(backgroundColor))
-                          << "\\green" << static_cast<int>(GetGValue(backgroundColor))
-                          << "\\blue" << static_cast<int>(GetBValue(backgroundColor))
-                          << ";";
-        colorMap[backgroundColor] = nextColorIndex++;
+
+        const auto getColorTableIndex = [&](const COLORREF color) -> size_t {
+            // Exclude the 0 index for the default color, and start with 1.
+
+            const auto [it, inserted] = colorMap.emplace(color, colorMap.size() + 1);
+            if (inserted)
+            {
+                colorTableBuilder << "\\red" << static_cast<int>(GetRValue(color))
+                                  << "\\green" << static_cast<int>(GetGValue(color))
+                                  << "\\blue" << static_cast<int>(GetBValue(color))
+                                  << ";";
+            }
+            return it->second;
+        };
 
         // content
         std::ostringstream contentBuilder;
@@ -2246,7 +2263,12 @@ std::string TextBuffer::GenRTF(const TextAndColor& rows, const int fontHeightPoi
         // \fs specifies font size in half-points i.e. \fs20 results in a font size
         // of 10 pts. That's why, font size is multiplied by 2 here.
         contentBuilder << "\\pard\\slmult1\\f0\\fs" << std::to_string(2 * fontHeightPoints)
-                       << "\\highlight1"
+                       // Set the background color for the page. But, the
+                       // standard way (\cbN) to do this isn't supported in Word.
+                       // However, the following control words sequence works
+                       // in Word (and other RTF editors also) for applying the
+                       // text background color. See: Spec 1.9.1, Pg. 23.
+                       << "\\chshdng0\\chcbpat" << getColorTableIndex(backgroundColor)
                        << " ";
 
         std::optional<COLORREF> fgColor = std::nullopt;
@@ -2296,43 +2318,8 @@ std::string TextBuffer::GenRTF(const TextAndColor& rows, const int fontHeightPoi
                 if (colorChanged)
                 {
                     writeAccumulatedChars(false);
-
-                    auto bkColorIndex = 0;
-                    if (colorMap.find(bkColor.value()) != colorMap.end())
-                    {
-                        // color already exists in the map, just retrieve the index
-                        bkColorIndex = colorMap[bkColor.value()];
-                    }
-                    else
-                    {
-                        // color not present in the map, so add it
-                        colorTableBuilder << "\\red" << static_cast<int>(GetRValue(bkColor.value()))
-                                          << "\\green" << static_cast<int>(GetGValue(bkColor.value()))
-                                          << "\\blue" << static_cast<int>(GetBValue(bkColor.value()))
-                                          << ";";
-                        colorMap[bkColor.value()] = nextColorIndex;
-                        bkColorIndex = nextColorIndex++;
-                    }
-
-                    auto fgColorIndex = 0;
-                    if (colorMap.find(fgColor.value()) != colorMap.end())
-                    {
-                        // color already exists in the map, just retrieve the index
-                        fgColorIndex = colorMap[fgColor.value()];
-                    }
-                    else
-                    {
-                        // color not present in the map, so add it
-                        colorTableBuilder << "\\red" << static_cast<int>(GetRValue(fgColor.value()))
-                                          << "\\green" << static_cast<int>(GetGValue(fgColor.value()))
-                                          << "\\blue" << static_cast<int>(GetBValue(fgColor.value()))
-                                          << ";";
-                        colorMap[fgColor.value()] = nextColorIndex;
-                        fgColorIndex = nextColorIndex++;
-                    }
-
-                    contentBuilder << "\\highlight" << bkColorIndex
-                                   << "\\cf" << fgColorIndex
+                    contentBuilder << "\\chshdng0\\chcbpat" << getColorTableIndex(bkColor.value())
+                                   << "\\cf" << getColorTableIndex(fgColor.value())
                                    << " ";
                 }