Fix horizontal scrolling issue after toggling softwrap on/off

Fixes #645

internal/buffer/buffer.go

@@ -189,6 +189,13 @@ type Buffer struct {
 	cursors     []*Cursor
 	curCursor   int
 	StartCursor Loc
+
+	// OptionCallback is called after a buffer option value is changed.
+	// The display module registers its OptionCallback to ensure the buffer window
+	// is properly updated when needed. This is a workaround for the fact that
+	// the buffer module cannot directly call the display's API (it would mean
+	// a circular dependency between packages).
+	OptionCallback func(option string, nativeValue interface{})
 }
 
 // NewBufferFromFileAtLoc opens a new buffer with a given cursor location

internal/buffer/settings.go

@@ -41,6 +41,10 @@ func (b *Buffer) SetOptionNative(option string, nativeValue interface{}) error {
 		b.Type.Readonly = nativeValue.(bool)
 	}
 
+	if b.OptionCallback != nil {
+		b.OptionCallback(option, nativeValue)
+	}
+
 	return nil
 }
 

internal/display/bufwindow.go

@@ -35,7 +35,8 @@ type BufWindow struct {
 func NewBufWindow(x, y, width, height int, buf *buffer.Buffer) *BufWindow {
 	w := new(BufWindow)
 	w.View = new(View)
-	w.X, w.Y, w.Width, w.Height, w.Buf = x, y, width, height, buf
+	w.X, w.Y, w.Width, w.Height = x, y, width, height
+	w.SetBuffer(buf)
 	w.active = true
 
 	w.sline = NewStatusLine(w)
@@ -45,6 +46,16 @@ func NewBufWindow(x, y, width, height int, buf *buffer.Buffer) *BufWindow {
 
 func (w *BufWindow) SetBuffer(b *buffer.Buffer) {
 	w.Buf = b
+	b.OptionCallback = func(option string, nativeValue interface{}) {
+		if option == "softwrap" {
+			if nativeValue.(bool) {
+				w.StartCol = 0
+			} else {
+				w.StartLine.Row = 0
+			}
+			w.Relocate()
+		}
+	}
 }
 
 func (w *BufWindow) GetView() *View {