fix: renderer: keep a separate count of lines rendered in the alt screen

We need to keep a separate count of lines rendered in the alt screen and
inline mode. This is because when we enter alt screen mode, the cursor
position is saved and the alt screen is cleared. Now when we exit the
alt screen mode, the cursor position is restored so we don't need to
move the cursor to the beginning of the section that we rendered.

To fix this, we keep a separate count of lines rendered in the alt screen
mode and inline mode.

Related: #1013
Fixes: #1241
Fixes: #1248

standard_renderer.go

@@ -36,6 +36,7 @@ type standardRenderer struct {
 	lastRender         string
 	lastRenderedLines  []string
 	linesRendered      int
+	altLinesRendered   int
 	useANSICompressor  bool
 	once               sync.Once
 
@@ -170,7 +171,9 @@ func (r *standardRenderer) flush() {
 	buf := &bytes.Buffer{}
 
 	// Moving to the beginning of the section, that we rendered.
-	if r.linesRendered > 1 {
+	if r.altScreenActive {
+		buf.WriteString(ansi.HomeCursorPosition)
+	} else if r.linesRendered > 1 {
 		buf.WriteString(ansi.CursorUp(r.linesRendered - 1))
 	}
 
@@ -255,20 +258,29 @@ func (r *standardRenderer) flush() {
 		}
 	}
 
+	lastLinesRendered := r.linesRendered
+	if r.altScreenActive {
+		lastLinesRendered = r.altLinesRendered
+	}
+
 	// Clearing left over content from last render.
-	if r.linesRendered > len(newLines) {
+	if lastLinesRendered > len(newLines) {
 		buf.WriteString(ansi.EraseScreenBelow)
 	}
 
-	r.linesRendered = len(newLines)
+	if r.altScreenActive {
+		r.altLinesRendered = len(newLines)
+	} else {
+		r.linesRendered = len(newLines)
+	}
 
 	// Make sure the cursor is at the start of the last line to keep rendering
 	// behavior consistent.
 	if r.altScreenActive {
 		// This case fixes a bug in macOS terminal. In other terminals the
 		// other case seems to do the job regardless of whether or not we're
 		// using the full terminal window.
-		buf.WriteString(ansi.SetCursorPosition(0, r.linesRendered))
+		buf.WriteString(ansi.SetCursorPosition(0, len(newLines)))
 	} else {
 		buf.WriteString(ansi.CursorLeft(r.width))
 	}
@@ -352,6 +364,9 @@ func (r *standardRenderer) enterAltScreen() {
 		r.execute(ansi.ShowCursor)
 	}
 
+	// Entering the alt screen resets the lines rendered count.
+	r.altLinesRendered = 0
+
 	r.repaint()
 }