Teach PSReadline to not force the background color during render (#1626)

Upcoming versions of the Windows Console will be able to differentiate
a color set by the Win32 API from a color set by VT. This code
transforms a Win32 color into a VT color, and then uses that VT color
during rendering. When an enlightened console host receives this VT, it
may handle it differently (translate it differently, reverse it
differently, store it differently). Use `39;49` for the default foreground/background.

We'll also teach the various console implementations how to
handle it.

PSReadLine/PlatformWindows.cs

@@ -416,6 +416,7 @@ internal class LegacyWin32Console : VirtualTerminal
             {45, () => Console.BackgroundColor = ConsoleColor.DarkMagenta},
             {43, () => Console.BackgroundColor = ConsoleColor.DarkYellow},
             {47, () => Console.BackgroundColor = ConsoleColor.Gray},
+            {49, () => Console.BackgroundColor = InitialBG},
             {100, () => Console.BackgroundColor = ConsoleColor.DarkGray},
             {104, () => Console.BackgroundColor = ConsoleColor.Blue},
             {102, () => Console.BackgroundColor = ConsoleColor.Green},
@@ -432,6 +433,7 @@ internal class LegacyWin32Console : VirtualTerminal
             {35, () => Console.ForegroundColor = ConsoleColor.DarkMagenta},
             {33, () => Console.ForegroundColor = ConsoleColor.DarkYellow},
             {37, () => Console.ForegroundColor = ConsoleColor.Gray},
+            {39, () => Console.ForegroundColor = InitialFG},
             {90, () => Console.ForegroundColor = ConsoleColor.DarkGray},
             {94, () => Console.ForegroundColor = ConsoleColor.Blue},
             {92, () => Console.ForegroundColor = ConsoleColor.Green},

PSReadLine/Render.cs

@@ -122,8 +122,7 @@ private void Render()
 
         private void ForceRender()
         {
-            var defaultColor = VTColorUtils.MapColorToEscapeSequence(_console.ForegroundColor, isBackground: false) +
-                               VTColorUtils.MapColorToEscapeSequence(_console.BackgroundColor, isBackground: true);
+            var defaultColor = "\x1b[39;49m";
 
             // Geneate a sequence of logical lines with escape sequences for coloring.
             int logicalLineCount = GenerateRender(defaultColor);

test/MockConsole.cs

@@ -313,6 +313,7 @@ private static void ToggleNegative(TestConsole c, bool b)
             {"45", c => c.BackgroundColor = ConsoleColor.DarkMagenta},
             {"43", c => c.BackgroundColor = ConsoleColor.DarkYellow},
             {"47", c => c.BackgroundColor = ConsoleColor.Gray},
+            {"49", c => c.BackgroundColor = DefaultBackground},
             {"100", c => c.BackgroundColor = ConsoleColor.DarkGray},
             {"104", c => c.BackgroundColor = ConsoleColor.Blue},
             {"102", c => c.BackgroundColor = ConsoleColor.Green},
@@ -329,6 +330,7 @@ private static void ToggleNegative(TestConsole c, bool b)
             {"35", c => c.ForegroundColor = ConsoleColor.DarkMagenta},
             {"33", c => c.ForegroundColor = ConsoleColor.DarkYellow},
             {"37", c => c.ForegroundColor = ConsoleColor.Gray},
+            {"39", c => c.ForegroundColor = DefaultForeground},
             {"90", c => c.ForegroundColor = ConsoleColor.DarkGray},
             {"94", c => c.ForegroundColor = ConsoleColor.Blue},
             {"92", c => c.ForegroundColor = ConsoleColor.Green},