feat(render): improve renderer; remove flickering

examples/simple/testdata/TestApp.golden

@@ -1,5 +1,5 @@
-[?25l[?2004hHi. This program will exit in 10 seconds.
-
-To quit sooner press ctrl-c, or press ctrl-z to suspend...
-Hi. This program will exit in 9 seconds.
+[?25l[?2004hHi. This program will exit in 10 seconds.
+
+To quit sooner press ctrl-c, or press ctrl-z to suspend...
+Hi. This program will exit in 9 seconds.
 [?2004l[?25h[?1002l[?1003l[?1006l

screen_test.go

@@ -14,47 +14,47 @@ func TestClearMsg(t *testing.T) {
 		{
 			name:     "clear_screen",
 			cmds:     []Cmd{ClearScreen},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[2J\x1b[1;1H\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[2J\x1b[1;1H\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "altscreen",
 			cmds:     []Cmd{EnterAltScreen, ExitAltScreen},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?1049h\x1b[2J\x1b[1;1H\x1b[?25l\x1b[?1049l\x1b[?25l\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?1049h\x1b[2J\x1b[1;1H\x1b[?25l\x1b[?1049l\x1b[?25l\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "altscreen_autoexit",
 			cmds:     []Cmd{EnterAltScreen},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?1049h\x1b[2J\x1b[1;1H\x1b[?25l\rsuccess\r\n\x1b[2;0H\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l\x1b[?1049l\x1b[?25h",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?1049h\x1b[2J\x1b[1;1H\x1b[?25l\rsuccess\x1b[0K\r\n\x1b[0K\x1b[2;0H\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l\x1b[?1049l\x1b[?25h",
 		},
 		{
 			name:     "mouse_cellmotion",
 			cmds:     []Cmd{EnableMouseCellMotion},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?1002h\x1b[?1006h\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?1002h\x1b[?1006h\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "mouse_allmotion",
 			cmds:     []Cmd{EnableMouseAllMotion},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?1003h\x1b[?1006h\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?1003h\x1b[?1006h\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "mouse_disable",
 			cmds:     []Cmd{EnableMouseAllMotion, DisableMouse},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?1003h\x1b[?1006h\x1b[?1002l\x1b[?1003l\x1b[?1006l\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?1003h\x1b[?1006h\x1b[?1002l\x1b[?1003l\x1b[?1006l\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "cursor_hide",
 			cmds:     []Cmd{HideCursor},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?25l\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?25l\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "cursor_hideshow",
 			cmds:     []Cmd{HideCursor, ShowCursor},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?25l\x1b[?25h\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?25l\x1b[?25h\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 		{
 			name:     "bp_stop_start",
 			cmds:     []Cmd{DisableBracketedPaste, EnableBracketedPaste},
-			expected: "\x1b[?25l\x1b[?2004h\x1b[?2004l\x1b[?2004h\rsuccess\r\n\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
+			expected: "\x1b[?25l\x1b[?2004h\x1b[?2004l\x1b[?2004h\rsuccess\x1b[0K\r\n\x1b[0K\x1b[D\x1b[2K\r\x1b[?2004l\x1b[?25h\x1b[?1002l\x1b[?1003l\x1b[?1006l",
 		},
 	}
 

standard_renderer.go

@@ -161,14 +161,20 @@ func (r *standardRenderer) flush() {
 	defer r.mtx.Unlock()
 
 	if r.buf.Len() == 0 || r.buf.String() == r.lastRender {
-		// Nothing to do
+		// Nothing to do.
 		return
 	}
 
-	// Output buffer
+	// Output buffer.
 	buf := &bytes.Buffer{}
 
+	// Moving to the beginning of the section, that we rendered.
+	if r.linesRendered > 1 {
+		buf.WriteString(ansi.CursorUp(r.linesRendered - 1))
+	}
+
 	newLines := strings.Split(r.buf.String(), "\n")
+	oldLines := strings.Split(r.lastRender, "\n")
 
 	// If we know the output's height, we can use it to determine how many
 	// lines we can render. We drop lines from the top of the render buffer if
@@ -179,95 +185,69 @@ func (r *standardRenderer) flush() {
 	}
 
 	numLinesThisFlush := len(newLines)
-	oldLines := strings.Split(r.lastRender, "\n")
-	skipLines := make(map[int]struct{})
 	flushQueuedMessages := len(r.queuedMessageLines) > 0 && !r.altScreenActive
 
-	// Clear any lines we painted in the last render.
-	if r.linesRendered > 0 {
-		for i := r.linesRendered - 1; i > 0; i-- {
-			// if we are clearing queued messages, we want to clear all lines, since
-			// printing messages allows for native terminal word-wrap, we
-			// don't have control over the queued lines
-			if flushQueuedMessages {
-				buf.WriteString(ansi.EraseEntireLine)
-			} else if (len(newLines) <= len(oldLines)) && (len(newLines) > i && len(oldLines) > i) && (newLines[i] == oldLines[i]) {
-				// If the number of lines we want to render hasn't increased and
-				// new line is the same as the old line we can skip rendering for
-				// this line as a performance optimization.
-				skipLines[i] = struct{}{}
-			} else if _, exists := r.ignoreLines[i]; !exists {
-				buf.WriteString(ansi.EraseEntireLine)
-			}
-
-			buf.WriteString(ansi.CursorUp1)
-		}
-
-		if _, exists := r.ignoreLines[0]; !exists {
-			// We need to return to the start of the line here to properly
-			// erase it. Going back the entire width of the terminal will
-			// usually be farther than we need to go, but terminal emulators
-			// will stop the cursor at the start of the line as a rule.
-			//
-			// We use this sequence in particular because it's part of the ANSI
-			// standard (whereas others are proprietary to, say, VT100/VT52).
-			// If cursor previous line (ESC[ + <n> + F) were better supported
-			// we could use that above to eliminate this step.
-			buf.WriteString(ansi.CursorLeft(r.width))
-			buf.WriteString(ansi.EraseEntireLine)
-		}
-	}
-
-	// Merge the set of lines we're skipping as a rendering optimization with
-	// the set of lines we've explicitly asked the renderer to ignore.
-	for k, v := range r.ignoreLines {
-		skipLines[k] = v
-	}
-
 	if flushQueuedMessages {
-		// Dump the lines we've queued up for printing
+		// Dump the lines we've queued up for printing.
 		for _, line := range r.queuedMessageLines {
+
+			// Removing previousy rendered content at the end of line.
+			line = line + ansi.EraseLineRight
+
 			_, _ = buf.WriteString(line)
 			_, _ = buf.WriteString("\r\n")
 		}
-		// clear the queued message lines
+		// Clear the queued message lines.
 		r.queuedMessageLines = []string{}
 	}
 
-	// Paint new lines
+	// Paint new lines.
 	for i := 0; i < len(newLines); i++ {
-		if _, skip := skipLines[i]; skip {
+		canSkip := !flushQueuedMessages && // Queuing messages triggers repaint -> we don't have access to previous frame content.
+			len(oldLines) > i && oldLines[i] == newLines[i] // Previously rendered line is the same.
+
+		if _, ignore := r.ignoreLines[i]; ignore || canSkip {
 			// Unless this is the last line, move the cursor down.
 			if i < len(newLines)-1 {
 				buf.WriteString(ansi.CursorDown1)
 			}
-		} else {
-			if i == 0 && r.lastRender == "" {
-				// On first render, reset the cursor to the start of the line
-				// before writing anything.
-				buf.WriteByte('\r')
-			}
+			continue
+		}
 
-			line := newLines[i]
-
-			// Truncate lines wider than the width of the window to avoid
-			// wrapping, which will mess up rendering. If we don't have the
-			// width of the window this will be ignored.
-			//
-			// Note that on Windows we only get the width of the window on
-			// program initialization, so after a resize this won't perform
-			// correctly (signal SIGWINCH is not supported on Windows).
-			if r.width > 0 {
-				line = ansi.Truncate(line, r.width, "")
-			}
+		if i == 0 && r.lastRender == "" {
+			// On first render, reset the cursor to the start of the line
+			// before writing anything.
+			buf.WriteByte('\r')
+		}
 
-			_, _ = buf.WriteString(line)
+		line := newLines[i]
 
-			if i < len(newLines)-1 {
-				_, _ = buf.WriteString("\r\n")
-			}
+		// Removing previousy rendered content at the end of line.
+		line = line + ansi.EraseLineRight
+
+		// Truncate lines wider than the width of the window to avoid
+		// wrapping, which will mess up rendering. If we don't have the
+		// width of the window this will be ignored.
+		//
+		// Note that on Windows we only get the width of the window on
+		// program initialization, so after a resize this won't perform
+		// correctly (signal SIGWINCH is not supported on Windows).
+		if r.width > 0 {
+			line = ansi.Truncate(line, r.width, "")
+		}
+
+		_, _ = buf.WriteString(line)
+
+		if i < len(newLines)-1 {
+			_, _ = buf.WriteString("\r\n")
 		}
 	}
+
+	// Clearing left over content from last render.
+	if r.linesRendered > numLinesThisFlush {
+		buf.WriteString(ansi.EraseScreenBelow)
+	}
+
 	r.linesRendered = numLinesThisFlush
 
 	// Make sure the cursor is at the start of the last line to keep rendering