options: Add truecolor to control the usage (#2867)

- `auto`: enable usage of true color if it is supported, otherwise disable it
- `on`: force usage of true color
- `off`: disable true color usage

Co-authored-by: Dmytro Maluka <dmitrymaluka@gmail.com>

internal/config/settings.go

@@ -36,6 +36,7 @@ var optionValidators = map[string]optionValidator{
 	"scrollmargin":    validateNonNegativeValue,
 	"scrollspeed":     validateNonNegativeValue,
 	"tabsize":         validatePositiveValue,
+	"truecolor":       validateChoice,
 }
 
 // a list of settings with pre-defined choices
@@ -46,6 +47,7 @@ var OptionChoices = map[string][]string{
 	"matchbracestyle": {"underline", "highlight"},
 	"multiopen":       {"tab", "hsplit", "vsplit"},
 	"reload":          {"prompt", "auto", "disabled"},
+	"truecolor":       {"auto", "on", "off"},
 }
 
 // a list of settings that can be globally and locally modified and their
@@ -99,6 +101,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"tabmovement":     false,
 	"tabsize":         float64(4),
 	"tabstospaces":    false,
+	"truecolor":       "auto",
 	"useprimary":      true,
 	"wordwrap":        false,
 }

internal/screen/screen.go

@@ -184,10 +184,13 @@ func Init() error {
 	drawChan = make(chan bool, 8)
 
 	// Should we enable true color?
-	truecolor := os.Getenv("MICRO_TRUECOLOR") == "1"
-
-	if !truecolor {
+	truecolor := config.GetGlobalOption("truecolor").(string)
+	if truecolor == "on" || (truecolor == "auto" && os.Getenv("MICRO_TRUECOLOR") == "1") {
+		os.Setenv("TCELL_TRUECOLOR", "enable")
+	} else if truecolor == "off" {
 		os.Setenv("TCELL_TRUECOLOR", "disable")
+	} else {
+		// For "auto", tcell already autodetects truecolor by default
 	}
 
 	var oldTerm string