Transpose color order when reading/writing legacy attributes.

src/buffer/out/TextAttribute.hpp

@@ -169,7 +169,7 @@ public:
 private:
     static constexpr TextColor s_LegacyIndexOrDefault(const BYTE requestedIndex, const BYTE defaultIndex)
     {
-        return requestedIndex == defaultIndex ? TextColor{} : TextColor{ requestedIndex, true };
+        return requestedIndex == defaultIndex ? TextColor{} : TextColor{ TextColor::TransposeLegacyIndex(requestedIndex), true };
     }
 
     static BYTE s_legacyDefaultForeground;

src/buffer/out/TextColor.cpp

@@ -32,7 +32,7 @@ constexpr std::array<BYTE, 256> CompressedRgbToIndex16 = {
 // A table mapping indexed colors from the 256-color palette,
 // down to one of the 16 colors in the legacy palette.
 constexpr std::array<BYTE, 256> Index256ToIndex16 = {
-     0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15,
+     0,  4,  2,  6,  1,  5,  3,  7,  8, 12, 10, 14,  9, 13, 11, 15,
      0,  1,  1,  1,  9,  9,  2,  1,  1,  1,  1,  1,  2,  2,  3,  3,
      3,  3,  2,  2, 11, 11,  3,  3, 10, 10, 11, 11, 11, 11, 10, 10,
     10, 10, 11, 11,  5,  5,  5,  5,  1,  1,  8,  8,  1,  1,  9,  9,
@@ -252,11 +252,7 @@ BYTE TextColor::GetLegacyIndex(const BYTE defaultIndex) const noexcept
     {
         return defaultIndex;
     }
-    else if (IsIndex16())
-    {
-        return GetIndex();
-    }
-    else if (IsIndex256())
+    else if (IsIndex16() || IsIndex256())
     {
         return til::at(Index256ToIndex16, GetIndex());
     }

src/buffer/out/TextColor.h

@@ -96,6 +96,16 @@ public:
 
     COLORREF GetRGB() const noexcept;
 
+    static constexpr BYTE TransposeLegacyIndex(const size_t index)
+    {
+        // When converting a 16-color index in the legacy Windows order to or
+        // from an ANSI-compatible order, we need to swap the bits in positions
+        // 0 and 2. We do this by XORing the index with 00000101, but only if
+        // one (but not both) of those bit positions is set.
+        const auto oneBitSet = (index ^ (index >> 2)) & 1;
+        return gsl::narrow_cast<BYTE>(index ^ oneBitSet ^ (oneBitSet << 2));
+    }
+
 private:
     union
     {