Added stable background mode

Added BackgroundMode:stable command to prevent the editor from changing the
background colour to indicate the mode. This is mostly useful for writing prose,
or documentation, where you're changing modes so frequently that the flashing
gets distracting. It can also be useful for people who delete many lines by
holding down the `d` key instead of using <n>dd.

COMMANDS.md

@@ -61,6 +61,11 @@ executing words.
 	to print line numbers relative to the cursor, or LineNumbers:absolute to
 	print absolute line numbers. LineNumbers with no arguments will toggle
 	the mode in the order off->relative->absolute->off
+* `BackgroundMode`: Sets whether or not the background should change colour to
+	indicate the keyboard mode. BackgroundMode:stable will prevent the
+	background from changing colour. Anything else will restore the default
+	mode where delete mode is red, and insert mode is a light greenish/blue
+	colour.
 
 ## Experimental Commands
 

actions/defaults/viewport.go

@@ -15,7 +15,7 @@ func init() {
 	actions.RegisterAction("TermWidth", TermWidth)
 	actions.RegisterAction("WarnAlpha", WarnAlpha)
 	actions.RegisterAction("LineNumbers", LineNumberMode)
-
+	actions.RegisterAction("BackgroundMode", BackgroundMode)
 }
 
 func TermWidth(args string, buff *demodel.CharBuffer, v demodel.Viewport) {
@@ -52,3 +52,12 @@ func LineNumberMode(args string, buff *demodel.CharBuffer, v demodel.Viewport) {
 		v.SetOption("RotateLineNumbers", nil)
 	}
 }
+
+func BackgroundMode(args string, buff *demodel.CharBuffer, v demodel.Viewport) {
+	switch args {
+	case "stable":
+		v.SetOption("BackgroundMode", viewer.StableBackground)
+	default:
+		v.SetOption("BackgroundMode", viewer.DefaultBackground)
+	}
+}

viewer/viewport.go

@@ -25,6 +25,18 @@ const (
 	AbsoluteLineNumbers
 )
 
+// BackgroundMode represents whether or not the viewport background should
+// change colour depending on the mode. It can be disabled by setting the
+// viewport mode to StableBackground.
+type BackgroundMode uint8
+
+const (
+	// Background is context-sensitive
+	DefaultBackground = BackgroundMode(iota)
+	// background does not change colour
+	StableBackground
+)
+
 type Viewport struct {
 	demodel.Map
 	demodel.Renderer
@@ -39,6 +51,7 @@ type Viewport struct {
 	warnalpha uint8
 
 	lineNumberMode LineNumberMode
+	BackgroundMode BackgroundMode
 }
 
 type RequestRerender struct{}
@@ -129,11 +142,17 @@ func (v *Viewport) SetOption(opt string, val interface{}) error {
 			v.lineNumberMode = NoLineNumbers
 		}
 		return nil
+	case "BackgroundMode":
+		if m, ok := val.(BackgroundMode); ok {
+			v.BackgroundMode = m
+		} else {
+			v.BackgroundMode = DefaultBackground
+		}
+		return nil
 	default:
 		return demodel.ErrUnsupportedOption
 	}
 }
-
 func mult(x, y fixed.Int26_6) fixed.Int26_6 {
 	// multiplying two fixed-point precision values requires shifting
 	// the decimal 6 places to get it back to the right place. This will

window.go

@@ -47,12 +47,19 @@ func (w *dewindow) realpaint(buf *demodel.CharBuffer, viewport *viewer.Viewport)
 
 	// Fill the buffer with the window background colour before
 	// drawing the web page on top of it.
-	switch viewport.GetKeyboardMode() {
-	case kbmap.InsertMode:
-		draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.InsertBackground}, image.ZP, draw.Src)
-	case kbmap.DeleteMode:
-		draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.DeleteBackground}, image.ZP, draw.Src)
-	default:
+	// This should logically be in the viewport code itself, but importing
+	// kbmap to switch on the mode sentinals would result in a cyclical
+	// import.
+	if viewport.BackgroundMode != viewer.StableBackground {
+		switch viewport.GetKeyboardMode() {
+		case kbmap.InsertMode:
+			draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.InsertBackground}, image.ZP, draw.Src)
+		case kbmap.DeleteMode:
+			draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.DeleteBackground}, image.ZP, draw.Src)
+		default:
+			draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.NormalBackground}, image.ZP, draw.Src)
+		}
+	} else {
 		draw.Draw(dst, dst.Bounds(), &image.Uniform{renderer.NormalBackground}, image.ZP, draw.Src)
 	}