fixes #606 Want ColorNone to preserve existing color(s)

While here, consolidate the use of the Fill() function from
CellBuffer (eliminating redundant code).

cell.go

@@ -1,4 +1,4 @@
-// Copyright 2022 The TCell Authors
+// Copyright 2023 The TCell Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use file except in compliance with the License.
@@ -31,7 +31,7 @@ type cell struct {
 	lock      bool
 }
 
-// CellBuffer represents a two dimensional array of character cells.
+// CellBuffer represents a two-dimensional array of character cells.
 // This is primarily intended for use by Screen implementors; it
 // contains much of the common code they need.  To create one, just
 // declare a variable of its type; no explicit initialization is necessary.
@@ -44,7 +44,9 @@ type CellBuffer struct {
 }
 
 // SetContent sets the contents (primary rune, combining runes,
-// and style) for a cell at a given location.
+// and style) for a cell at a given location.  If the background or
+// foreground of the style is set to ColorNone, then the respective
+// color is left un changed.
 func (cb *CellBuffer) SetContent(x int, y int,
 	mainc rune, combc []rune, style Style,
 ) {
@@ -61,6 +63,12 @@ func (cb *CellBuffer) SetContent(x int, y int,
 			c.width = runewidth.RuneWidth(mainc)
 		}
 		c.currMain = mainc
+		if style.fg == ColorNone {
+			style.fg = c.currStyle.fg
+		}
+		if style.bg == ColorNone {
+			style.bg = c.currStyle.bg
+		}
 		c.currStyle = style
 	}
 }
@@ -97,10 +105,9 @@ func (cb *CellBuffer) Invalidate() {
 	}
 }
 
-// Dirty checks if a character at the given location needs an
-// to be refreshed on the physical display.  This returns true
-// if the cell content is different since the last time it was
-// marked clean.
+// Dirty checks if a character at the given location needs to be
+// refreshed on the physical display.  This returns true if the cell
+// content is different since the last time it was marked clean.
 func (cb *CellBuffer) Dirty(x, y int) bool {
 	if x >= 0 && y >= 0 && x < cb.w && y < cb.h {
 		c := &cb.cells[(y*cb.w)+x]
@@ -204,12 +211,21 @@ func (cb *CellBuffer) Resize(w, h int) {
 // Fill fills the entire cell buffer array with the specified character
 // and style.  Normally choose ' ' to clear the screen.  This API doesn't
 // support combining characters, or characters with a width larger than one.
+// If either the foreground or background are ColorNone, then the respective
+// color is unchanged.
 func (cb *CellBuffer) Fill(r rune, style Style) {
 	for i := range cb.cells {
 		c := &cb.cells[i]
 		c.currMain = r
 		c.currComb = nil
-		c.currStyle = style
+		cs := style
+		if cs.fg == ColorNone {
+			cs.fg = c.currStyle.fg
+		}
+		if cs.bg == ColorNone {
+			cs.bg = c.currStyle.bg
+		}
+		c.currStyle = cs
 		c.width = 1
 	}
 }
@@ -224,7 +240,7 @@ func init() {
 		runewidth.DefaultCondition.EastAsianWidth = false
 	}
 
-	// For performance reasons, we create a lookup table.  However some users
+	// For performance reasons, we create a lookup table.  However, some users
 	// might be more memory conscious.  If that's you, set the TCELL_MINIMIZE
 	// environment variable.
 	if os.Getenv("TCELL_MINIMIZE") == "" {

color.go

@@ -1,4 +1,4 @@
-// Copyright 2020 The TCell Authors
+// Copyright 2023 The TCell Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use file except in compliance with the License.
@@ -839,6 +839,11 @@ const (
 	// ColorReset is used to indicate that the color should use the
 	// vanilla terminal colors.  (Basically go back to the defaults.)
 	ColorReset = ColorSpecial | iota
+
+	// ColorNone indicates that we should not change the color from
+	// whatever is already displayed.  This can only be used in limited
+	// circumstances.
+	ColorNone
 )
 
 // ColorNames holds the written names of colors. Useful to present a list of
@@ -1002,8 +1007,8 @@ func (c Color) IsRGB() bool {
 	return c&(ColorValid|ColorIsRGB) == (ColorValid | ColorIsRGB)
 }
 
-// CSS returns the CSS hex string ( #ABCDEF ) if valid 
-// if not a valid color returns empty string 
+// CSS returns the CSS hex string ( #ABCDEF ) if valid
+// if not a valid color returns empty string
 func (c Color) CSS() string {
 	if !c.Valid() {
 		return ""
@@ -1015,13 +1020,21 @@ func (c Color) CSS() string {
 // W3C name if it has one or the CSS hex string '#ABCDEF'
 func (c Color) String() string {
 	if !c.Valid() {
+		switch c {
+		case ColorNone:
+			return "none"
+		case ColorDefault:
+			return "default"
+		case ColorReset:
+			return "reset"
+		}
 		return ""
 	}
 	return c.Name(true)
 }
 
 // Name returns W3C name or an empty string if no arguments
-// if passed true as an argument it will falls back to 
+// if passed true as an argument it will falls back to
 // the CSS hex string if no W3C name found '#ABCDEF'
 func (c Color) Name(css ...bool) string {
 	for name, hex := range ColorNames {

color_test.go

@@ -1,4 +1,4 @@
-// Copyright 2018 The TCell Authors
+// Copyright 2023 The TCell Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use file except in compliance with the License.
@@ -46,7 +46,7 @@ func TestColorValues(t *testing.T) {
 }
 
 func TestColorFitting(t *testing.T) {
-	pal := []Color{}
+	var pal []Color
 	for i := 0; i < 255; i++ {
 		pal = append(pal, PaletteColor(i))
 	}
@@ -108,6 +108,17 @@ func TestColorNameLookup(t *testing.T) {
 			t.Errorf("TrueColor did not match")
 		}
 	}
+
+	// these colors only have strings (for debugging), you cannot use them to create a color
+	if ColorNone.String() != "none" {
+		t.Errorf("ColorNone did not match")
+	}
+	if ColorReset.String() != "reset" {
+		t.Errorf("ColorReset did not match")
+	}
+	if ColorDefault.String() != "default" {
+		t.Errorf("ColorDefault did not match")
+	}
 }
 
 func TestColorRGB(t *testing.T) {
@@ -132,3 +143,34 @@ func TestFromImageColor(t *testing.T) {
 		t.Errorf("%v is not 0x00FFFF", hex)
 	}
 }
+
+func TestColorNone(t *testing.T) {
+	s := mkTestScreen(t, "")
+	s.Init()
+	s.SetSize(80, 24)
+	st := StyleDefault.Foreground(ColorBlack).Background(ColorWhite)
+	s.Fill(' ', st)
+	if _, _, s1, _ := s.GetContent(0, 0); s1 != st {
+		t.Errorf("Wrong style! fg %s bg %s", s1.fg.String(), s1.bg.String())
+	}
+	st2 := st.Foreground(ColorNone).Background(ColorNone)
+	s.Fill('X', st2)
+	if _, _, s1, _ := s.GetContent(0, 0); s1 != st {
+		t.Errorf("Wrong style! fg %s bg %s", s1.fg.String(), s1.bg.String())
+	}
+	red := st.Foreground(ColorRed).Background(ColorNone)
+	s.SetContent(1, 0, ' ', nil, red)
+	if _, _, s1, _ := s.GetContent(1, 0); s1 != red.Background(st.bg) {
+		t.Errorf("Wrong style! fg %s bg %s", s1.fg.String(), s1.bg.String())
+	}
+	if _, _, s1, _ := s.GetContent(0, 0); s1 != st {
+		t.Errorf("Wrong style! fg %s bg %s", s1.fg.String(), s1.bg.String())
+	}
+	pink := st.Background(ColorPink).Foreground(ColorNone)
+	s.SetContent(1, 0, ' ', nil, pink)
+	combined := pink.Foreground(ColorRed)
+
+	if _, _, s1, _ := s.GetContent(1, 0); s1 != combined {
+		t.Errorf("Wrong style! fg %s bg %s", s1.fg.String(), s1.bg.String())
+	}
+}

screen.go

@@ -376,13 +376,7 @@ func (b *baseScreen) Clear() {
 func (b *baseScreen) Fill(r rune, style Style) {
 	cb := b.GetCells()
 	b.Lock()
-	for i := range cb.cells {
-		c := &cb.cells[i]
-		c.currMain = r
-		c.currComb = nil
-		c.currStyle = style
-		c.width = 1
-	}
+	cb.Fill(r, style)
 	b.Unlock()
 }