Implement moving cursor up/down within a wrapped line

Modified behavior of CursorUp, CursorDown, CursorPageUp etc:
if softwrap is enabled, cursor moves by visual lines, not logical lines.

TODO: implement it also for Home and End keys: move cursor to the
visual start or end of a line. I haven't implemented it for now, because
I'm not sure what should be the behavior of StartOfTextToggle then
(considering that Home key is bound to StartOfTextToggle by default).

Fixes #1598

internal/action/actions.go

@@ -123,10 +123,49 @@ func (h *BufPane) Center() bool {
 	return true
 }
 
+// MoveCursorUp is not an action
+func (h *BufPane) MoveCursorUp(n int) {
+	if !h.Buf.Settings["softwrap"].(bool) {
+		h.Cursor.UpN(n)
+	} else {
+		vloc := h.VLocFromLoc(h.Cursor.Loc)
+		sloc := h.Scroll(vloc.SLoc, -n)
+		if sloc == vloc.SLoc {
+			// we are at the beginning of buffer
+			h.Cursor.Loc = h.Buf.Start()
+			h.Cursor.LastVisualX = 0
+		} else {
+			vloc.SLoc = sloc
+			vloc.VisualX = h.Cursor.LastVisualX
+			h.Cursor.Loc = h.LocFromVLoc(vloc)
+		}
+	}
+}
+
+// MoveCursorDown is not an action
+func (h *BufPane) MoveCursorDown(n int) {
+	if !h.Buf.Settings["softwrap"].(bool) {
+		h.Cursor.DownN(n)
+	} else {
+		vloc := h.VLocFromLoc(h.Cursor.Loc)
+		sloc := h.Scroll(vloc.SLoc, n)
+		if sloc == vloc.SLoc {
+			// we are at the end of buffer
+			h.Cursor.Loc = h.Buf.End()
+			vloc = h.VLocFromLoc(h.Cursor.Loc)
+			h.Cursor.LastVisualX = vloc.VisualX
+		} else {
+			vloc.SLoc = sloc
+			vloc.VisualX = h.Cursor.LastVisualX
+			h.Cursor.Loc = h.LocFromVLoc(vloc)
+		}
+	}
+}
+
 // CursorUp moves the cursor up
 func (h *BufPane) CursorUp() bool {
 	h.Cursor.Deselect(true)
-	h.Cursor.Up()
+	h.MoveCursorUp(1)
 	h.Relocate()
 	return true
 }
@@ -134,7 +173,7 @@ func (h *BufPane) CursorUp() bool {
 // CursorDown moves the cursor down
 func (h *BufPane) CursorDown() bool {
 	h.Cursor.Deselect(true)
-	h.Cursor.Down()
+	h.MoveCursorDown(1)
 	h.Relocate()
 	return true
 }
@@ -212,7 +251,7 @@ func (h *BufPane) SelectUp() bool {
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.Cursor.Up()
+	h.MoveCursorUp(1)
 	h.Cursor.SelectTo(h.Cursor.Loc)
 	h.Relocate()
 	return true
@@ -223,7 +262,7 @@ func (h *BufPane) SelectDown() bool {
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.Cursor.Down()
+	h.MoveCursorDown(1)
 	h.Cursor.SelectTo(h.Cursor.Loc)
 	h.Relocate()
 	return true
@@ -1274,7 +1313,7 @@ func (h *BufPane) SelectPageUp() bool {
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.Cursor.UpN(h.BufHeight())
+	h.MoveCursorUp(h.BufHeight())
 	h.Cursor.SelectTo(h.Cursor.Loc)
 	h.Relocate()
 	return true
@@ -1285,7 +1324,7 @@ func (h *BufPane) SelectPageDown() bool {
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.Cursor.DownN(h.BufHeight())
+	h.MoveCursorDown(h.BufHeight())
 	h.Cursor.SelectTo(h.Cursor.Loc)
 	h.Relocate()
 	return true
@@ -1300,7 +1339,7 @@ func (h *BufPane) CursorPageUp() bool {
 		h.Cursor.ResetSelection()
 		h.Cursor.StoreVisualX()
 	}
-	h.Cursor.UpN(h.BufHeight())
+	h.MoveCursorUp(h.BufHeight())
 	h.Relocate()
 	return true
 }
@@ -1314,7 +1353,7 @@ func (h *BufPane) CursorPageDown() bool {
 		h.Cursor.ResetSelection()
 		h.Cursor.StoreVisualX()
 	}
-	h.Cursor.DownN(h.BufHeight())
+	h.MoveCursorDown(h.BufHeight())
 	h.Relocate()
 	return true
 }

internal/buffer/buffer.go

@@ -196,6 +196,12 @@ type Buffer struct {
 	// the buffer module cannot directly call the display's API (it would mean
 	// a circular dependency between packages).
 	OptionCallback func(option string, nativeValue interface{})
+
+	// The display module registers its own GetVisualX function for getting
+	// the correct visual x location of a cursor when softwrap is used.
+	// This is hacky. Maybe it would be better to move all the visual x logic
+	// from buffer to display, but it would require rewriting a lot of code.
+	GetVisualX func(loc Loc) int
 }
 
 // NewBufferFromFileAtLoc opens a new buffer with a given cursor location

internal/buffer/cursor.go

@@ -67,6 +67,10 @@ func (c *Cursor) GotoLoc(l Loc) {
 
 // GetVisualX returns the x value of the cursor in visual spaces
 func (c *Cursor) GetVisualX() int {
+	if c.buf.GetVisualX != nil {
+		return c.buf.GetVisualX(c.Loc)
+	}
+
 	if c.X <= 0 {
 		c.X = 0
 		return 0

internal/display/bufwindow.go

@@ -54,8 +54,15 @@ func (w *BufWindow) SetBuffer(b *buffer.Buffer) {
 				w.StartLine.Row = 0
 			}
 			w.Relocate()
+
+			for _, c := range w.Buf.GetCursors() {
+				c.LastVisualX = c.GetVisualX()
+			}
 		}
 	}
+	b.GetVisualX = func(loc buffer.Loc) int {
+		return w.VLocFromLoc(loc).VisualX
+	}
 }
 
 func (w *BufWindow) GetView() *View {
@@ -68,7 +75,15 @@ func (w *BufWindow) SetView(view *View) {
 
 func (w *BufWindow) Resize(width, height int) {
 	w.Width, w.Height = width, height
+	w.updateDisplayInfo()
+
 	w.Relocate()
+
+	if w.Buf.Settings["softwrap"].(bool) {
+		for _, c := range w.Buf.GetCursors() {
+			c.LastVisualX = c.GetVisualX()
+		}
+	}
 }
 
 func (w *BufWindow) SetActive(b bool) {