feat(terminal)!: add inline viewport (#114)

Co-authored-by: Florian Dehau <work@fdehau.com>
Co-authored-by: Orhun Parmaksız <orhunparmaksiz@gmail.com>

Cargo.toml

@@ -96,3 +96,7 @@ required-features = ["crossterm"]
 [[example]]
 name = "user_input"
 required-features = ["crossterm"]
+
+[[example]]
+name = "inline"
+required-features = ["crossterm"]

examples/inline.rs

@@ -0,0 +1,292 @@
+use rand::distributions::{Distribution, Uniform};
+use ratatui::{
+    backend::{Backend, CrosstermBackend},
+    layout::{Alignment, Constraint, Direction, Layout, Rect},
+    style::{Color, Modifier, Style},
+    symbols,
+    text::{Span, Spans},
+    widgets::{Block, Gauge, LineGauge, List, ListItem, Paragraph, Widget},
+    Frame, Terminal, TerminalOptions, Viewport,
+};
+use std::{
+    collections::{BTreeMap, VecDeque},
+    error::Error,
+    io,
+    sync::mpsc,
+    thread,
+    time::{Duration, Instant},
+};
+
+const NUM_DOWNLOADS: usize = 10;
+
+type DownloadId = usize;
+type WorkerId = usize;
+
+enum Event {
+    Input(crossterm::event::KeyEvent),
+    Tick,
+    Resize,
+    DownloadUpdate(WorkerId, DownloadId, f64),
+    DownloadDone(WorkerId, DownloadId),
+}
+
+struct Downloads {
+    pending: VecDeque<Download>,
+    in_progress: BTreeMap<WorkerId, DownloadInProgress>,
+}
+
+impl Downloads {
+    fn next(&mut self, worker_id: WorkerId) -> Option<Download> {
+        match self.pending.pop_front() {
+            Some(d) => {
+                self.in_progress.insert(
+                    worker_id,
+                    DownloadInProgress {
+                        id: d.id,
+                        started_at: Instant::now(),
+                        progress: 0.0,
+                    },
+                );
+                Some(d)
+            }
+            None => None,
+        }
+    }
+}
+
+struct DownloadInProgress {
+    id: DownloadId,
+    started_at: Instant,
+    progress: f64,
+}
+
+struct Download {
+    id: DownloadId,
+    size: usize,
+}
+
+struct Worker {
+    id: WorkerId,
+    tx: mpsc::Sender<Download>,
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    crossterm::terminal::enable_raw_mode()?;
+    let stdout = io::stdout();
+    let backend = CrosstermBackend::new(stdout);
+    let mut terminal = Terminal::with_options(
+        backend,
+        TerminalOptions {
+            viewport: Viewport::Inline(8),
+        },
+    )?;
+
+    let (tx, rx) = mpsc::channel();
+    input_handling(tx.clone());
+    let workers = workers(tx);
+    let mut downloads = downloads();
+
+    for w in &workers {
+        let d = downloads.next(w.id).unwrap();
+        w.tx.send(d).unwrap();
+    }
+
+    run_app(&mut terminal, workers, downloads, rx)?;
+
+    crossterm::terminal::disable_raw_mode()?;
+    terminal.clear()?;
+
+    Ok(())
+}
+
+fn input_handling(tx: mpsc::Sender<Event>) {
+    let tick_rate = Duration::from_millis(200);
+    thread::spawn(move || {
+        let mut last_tick = Instant::now();
+        loop {
+            // poll for tick rate duration, if no events, sent tick event.
+            let timeout = tick_rate
+                .checked_sub(last_tick.elapsed())
+                .unwrap_or_else(|| Duration::from_secs(0));
+            if crossterm::event::poll(timeout).unwrap() {
+                match crossterm::event::read().unwrap() {
+                    crossterm::event::Event::Key(key) => tx.send(Event::Input(key)).unwrap(),
+                    crossterm::event::Event::Resize(_, _) => tx.send(Event::Resize).unwrap(),
+                    _ => {}
+                };
+            }
+            if last_tick.elapsed() >= tick_rate {
+                tx.send(Event::Tick).unwrap();
+                last_tick = Instant::now();
+            }
+        }
+    });
+}
+
+fn workers(tx: mpsc::Sender<Event>) -> Vec<Worker> {
+    (0..4)
+        .map(|id| {
+            let (worker_tx, worker_rx) = mpsc::channel::<Download>();
+            let tx = tx.clone();
+            thread::spawn(move || {
+                while let Ok(download) = worker_rx.recv() {
+                    let mut remaining = download.size;
+                    while remaining > 0 {
+                        let wait = (remaining as u64).min(10);
+                        thread::sleep(Duration::from_millis(wait * 10));
+                        remaining = remaining.saturating_sub(10);
+                        let progress = (download.size - remaining) * 100 / download.size;
+                        tx.send(Event::DownloadUpdate(id, download.id, progress as f64))
+                            .unwrap();
+                    }
+                    tx.send(Event::DownloadDone(id, download.id)).unwrap();
+                }
+            });
+            Worker { id, tx: worker_tx }
+        })
+        .collect()
+}
+
+fn downloads() -> Downloads {
+    let distribution = Uniform::new(0, 1000);
+    let mut rng = rand::thread_rng();
+    let pending = (0..NUM_DOWNLOADS)
+        .map(|id| {
+            let size = distribution.sample(&mut rng);
+            Download { id, size }
+        })
+        .collect();
+    Downloads {
+        pending,
+        in_progress: BTreeMap::new(),
+    }
+}
+
+fn run_app<B: Backend>(
+    terminal: &mut Terminal<B>,
+    workers: Vec<Worker>,
+    mut downloads: Downloads,
+    rx: mpsc::Receiver<Event>,
+) -> Result<(), Box<dyn Error>> {
+    let mut redraw = true;
+    loop {
+        if redraw {
+            terminal.draw(|f| ui(f, &downloads))?;
+        }
+        redraw = true;
+
+        match rx.recv()? {
+            Event::Input(event) => {
+                if event.code == crossterm::event::KeyCode::Char('q') {
+                    break;
+                }
+            }
+            Event::Resize => {
+                terminal.autoresize()?;
+            }
+            Event::Tick => {}
+            Event::DownloadUpdate(worker_id, _download_id, progress) => {
+                let download = downloads.in_progress.get_mut(&worker_id).unwrap();
+                download.progress = progress;
+                redraw = false
+            }
+            Event::DownloadDone(worker_id, download_id) => {
+                let download = downloads.in_progress.remove(&worker_id).unwrap();
+                terminal.insert_before(1, |buf| {
+                    Paragraph::new(Spans::from(vec![
+                        Span::from("Finished "),
+                        Span::styled(
+                            format!("download {}", download_id),
+                            Style::default().add_modifier(Modifier::BOLD),
+                        ),
+                        Span::from(format!(
+                            " in {}ms",
+                            download.started_at.elapsed().as_millis()
+                        )),
+                    ]))
+                    .render(buf.area, buf);
+                })?;
+                match downloads.next(worker_id) {
+                    Some(d) => workers[worker_id].tx.send(d).unwrap(),
+                    None => {
+                        if downloads.in_progress.is_empty() {
+                            terminal.insert_before(1, |buf| {
+                                Paragraph::new("Done !").render(buf.area, buf);
+                            })?;
+                            break;
+                        }
+                    }
+                };
+            }
+        };
+    }
+    Ok(())
+}
+
+fn ui<B: Backend>(f: &mut Frame<B>, downloads: &Downloads) {
+    let size = f.size();
+
+    let block = Block::default()
+        .title("Progress")
+        .title_alignment(Alignment::Center);
+    f.render_widget(block, size);
+
+    let chunks = Layout::default()
+        .constraints(vec![Constraint::Length(2), Constraint::Length(4)])
+        .margin(1)
+        .split(size);
+
+    // total progress
+    let done = NUM_DOWNLOADS - downloads.pending.len() - downloads.in_progress.len();
+    let progress = LineGauge::default()
+        .gauge_style(Style::default().fg(Color::Blue))
+        .label(format!("{}/{}", done, NUM_DOWNLOADS))
+        .ratio(done as f64 / NUM_DOWNLOADS as f64);
+    f.render_widget(progress, chunks[0]);
+
+    let chunks = Layout::default()
+        .direction(Direction::Horizontal)
+        .constraints(vec![Constraint::Percentage(20), Constraint::Percentage(80)])
+        .split(chunks[1]);
+
+    // in progress downloads
+    let items: Vec<ListItem> = downloads
+        .in_progress
+        .values()
+        .map(|download| {
+            ListItem::new(Spans::from(vec![
+                Span::raw(symbols::DOT),
+                Span::styled(
+                    format!(" download {:>2}", download.id),
+                    Style::default()
+                        .fg(Color::LightGreen)
+                        .add_modifier(Modifier::BOLD),
+                ),
+                Span::raw(format!(
+                    " ({}ms)",
+                    download.started_at.elapsed().as_millis()
+                )),
+            ]))
+        })
+        .collect();
+    let list = List::new(items);
+    f.render_widget(list, chunks[0]);
+
+    for (i, (_, download)) in downloads.in_progress.iter().enumerate() {
+        let gauge = Gauge::default()
+            .gauge_style(Style::default().fg(Color::Yellow))
+            .ratio(download.progress / 100.0);
+        if chunks[1].top().saturating_add(i as u16) > size.bottom() {
+            continue;
+        }
+        f.render_widget(
+            gauge,
+            Rect {
+                x: chunks[1].left(),
+                y: chunks[1].top().saturating_add(i as u16),
+                width: chunks[1].width,
+                height: 1,
+            },
+        );
+    }
+}

src/backend/crossterm.rs

@@ -1,5 +1,5 @@
 use crate::{
-    backend::Backend,
+    backend::{Backend, ClearType},
     buffer::Cell,
     layout::Rect,
     style::{Color, Modifier},
@@ -11,7 +11,7 @@ use crossterm::{
         Attribute as CAttribute, Color as CColor, Print, SetAttribute, SetBackgroundColor,
         SetForegroundColor,
     },
-    terminal::{self, Clear, ClearType},
+    terminal::{self, Clear},
 };
 use std::io::{self, Write};
 
@@ -107,7 +107,27 @@ where
     }
 
     fn clear(&mut self) -> io::Result<()> {
-        map_error(execute!(self.buffer, Clear(ClearType::All)))
+        self.clear_region(ClearType::All)
+    }
+
+    fn clear_region(&mut self, clear_type: ClearType) -> io::Result<()> {
+        map_error(execute!(
+            self.buffer,
+            Clear(match clear_type {
+                ClearType::All => crossterm::terminal::ClearType::All,
+                ClearType::AfterCursor => crossterm::terminal::ClearType::FromCursorDown,
+                ClearType::BeforeCursor => crossterm::terminal::ClearType::FromCursorUp,
+                ClearType::CurrentLine => crossterm::terminal::ClearType::CurrentLine,
+                ClearType::UntilNewLine => crossterm::terminal::ClearType::UntilNewLine,
+            })
+        ))
+    }
+
+    fn append_lines(&mut self, n: u16) -> io::Result<()> {
+        for _ in 0..n {
+            map_error(queue!(self.buffer, Print("\n")))?;
+        }
+        self.buffer.flush()
     }
 
     fn size(&self) -> io::Result<Rect> {

src/backend/mod.rs

@@ -16,15 +16,48 @@ pub use self::crossterm::CrosstermBackend;
 mod test;
 pub use self::test::TestBackend;
 
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum ClearType {
+    All,
+    AfterCursor,
+    BeforeCursor,
+    CurrentLine,
+    UntilNewLine,
+}
+
 pub trait Backend {
     fn draw<'a, I>(&mut self, content: I) -> Result<(), io::Error>
     where
         I: Iterator<Item = (u16, u16, &'a Cell)>;
+
+    /// Insert `n` line breaks to the terminal screen
+    fn append_lines(&mut self, n: u16) -> io::Result<()> {
+        // to get around the unused warning
+        let _n = n;
+        Ok(())
+    }
+
     fn hide_cursor(&mut self) -> Result<(), io::Error>;
     fn show_cursor(&mut self) -> Result<(), io::Error>;
     fn get_cursor(&mut self) -> Result<(u16, u16), io::Error>;
     fn set_cursor(&mut self, x: u16, y: u16) -> Result<(), io::Error>;
+
+    /// Clears the whole terminal screen
     fn clear(&mut self) -> Result<(), io::Error>;
+
+    /// Clears a specific region of the terminal specified by the [`ClearType`] parameter
+    fn clear_region(&mut self, clear_type: ClearType) -> Result<(), io::Error> {
+        match clear_type {
+            ClearType::All => self.clear(),
+            ClearType::AfterCursor
+            | ClearType::BeforeCursor
+            | ClearType::CurrentLine
+            | ClearType::UntilNewLine => Err(io::Error::new(
+                io::ErrorKind::Other,
+                format!("clear_type [{clear_type:?}] not supported with this backend"),
+            )),
+        }
+    }
     fn size(&self) -> Result<Rect, io::Error>;
     fn flush(&mut self) -> Result<(), io::Error>;
 }

src/backend/termion.rs

@@ -1,5 +1,5 @@
-use super::Backend;
 use crate::{
+    backend::{Backend, ClearType},
     buffer::Cell,
     layout::Rect,
     style::{Color, Modifier},
@@ -42,10 +42,25 @@ impl<W> Backend for TermionBackend<W>
 where
     W: Write,
 {
-    /// Clears the entire screen and move the cursor to the top left of the screen
     fn clear(&mut self) -> io::Result<()> {
-        write!(self.stdout, "{}", termion::clear::All)?;
-        write!(self.stdout, "{}", termion::cursor::Goto(1, 1))?;
+        self.clear_region(ClearType::All)
+    }
+
+    fn clear_region(&mut self, clear_type: ClearType) -> io::Result<()> {
+        match clear_type {
+            ClearType::All => write!(self.stdout, "{}", termion::clear::All)?,
+            ClearType::AfterCursor => write!(self.stdout, "{}", termion::clear::AfterCursor)?,
+            ClearType::BeforeCursor => write!(self.stdout, "{}", termion::clear::BeforeCursor)?,
+            ClearType::CurrentLine => write!(self.stdout, "{}", termion::clear::CurrentLine)?,
+            ClearType::UntilNewLine => write!(self.stdout, "{}", termion::clear::UntilNewline)?,
+        };
+        self.stdout.flush()
+    }
+
+    fn append_lines(&mut self, n: u16) -> io::Result<()> {
+        for _ in 0..n {
+            writeln!(self.stdout)?;
+        }
         self.stdout.flush()
     }
 

src/buffer.rs

@@ -431,7 +431,6 @@ impl Buffer {
     pub fn diff<'a>(&self, other: &'a Buffer) -> Vec<(u16, u16, &'a Cell)> {
         let previous_buffer = &self.content;
         let next_buffer = &other.content;
-        let width = self.area.width;
 
         let mut updates: Vec<(u16, u16, &Cell)> = vec![];
         // Cells invalidated by drawing/replacing preceding multi-width characters:
@@ -441,8 +440,7 @@ impl Buffer {
         let mut to_skip: usize = 0;
         for (i, (current, previous)) in next_buffer.iter().zip(previous_buffer.iter()).enumerate() {
             if (current != previous || invalidated > 0) && to_skip == 0 {
-                let x = i as u16 % width;
-                let y = i as u16 / width;
+                let (x, y) = self.pos_of(i);
                 updates.push((x, y, &next_buffer[i]));
             }
 

src/terminal.rs

@@ -1,5 +1,5 @@
 use crate::{
-    backend::Backend,
+    backend::{Backend, ClearType},
     buffer::Buffer,
     layout::Rect,
     widgets::{StatefulWidget, Widget},
@@ -7,27 +7,10 @@ use crate::{
 use std::io;
 
 #[derive(Debug, Clone, PartialEq)]
-/// UNSTABLE
-enum ResizeBehavior {
-    Fixed,
-    Auto,
-}
-
-#[derive(Debug, Clone, PartialEq)]
-/// UNSTABLE
-pub struct Viewport {
-    area: Rect,
-    resize_behavior: ResizeBehavior,
-}
-
-impl Viewport {
-    /// UNSTABLE
-    pub fn fixed(area: Rect) -> Viewport {
-        Viewport {
-            area,
-            resize_behavior: ResizeBehavior::Fixed,
-        }
-    }
+pub enum Viewport {
+    Fullscreen,
+    Inline(u16),
+    Fixed(Rect),
 }
 
 #[derive(Debug, Clone, PartialEq)]
@@ -53,6 +36,12 @@ where
     hidden_cursor: bool,
     /// Viewport
     viewport: Viewport,
+    viewport_area: Rect,
+    /// Last known size of the terminal. Used to detect if the internal buffers have to be resized.
+    last_known_size: Rect,
+    /// Last known position of the cursor. Used to find the new area when the viewport is inlined
+    /// and the terminal resized.
+    last_known_cursor_pos: (u16, u16),
 }
 
 /// Represents a consistent terminal interface for rendering.
@@ -73,9 +62,9 @@ impl<'a, B> Frame<'a, B>
 where
     B: Backend,
 {
-    /// Terminal size, guaranteed not to change when rendering.
+    /// Frame size, guaranteed not to change when rendering.
     pub fn size(&self) -> Rect {
-        self.terminal.viewport.area
+        self.terminal.viewport_area
     }
 
     /// Render a [`Widget`] to the current buffer using [`Widget::render`].
@@ -173,29 +162,33 @@ where
     /// Wrapper around Terminal initialization. Each buffer is initialized with a blank string and
     /// default colors for the foreground and the background
     pub fn new(backend: B) -> io::Result<Terminal<B>> {
-        let size = backend.size()?;
         Terminal::with_options(
             backend,
             TerminalOptions {
-                viewport: Viewport {
-                    area: size,
-                    resize_behavior: ResizeBehavior::Auto,
-                },
+                viewport: Viewport::Fullscreen,
             },
         )
     }
 
-    /// UNSTABLE
-    pub fn with_options(backend: B, options: TerminalOptions) -> io::Result<Terminal<B>> {
+    pub fn with_options(mut backend: B, options: TerminalOptions) -> io::Result<Terminal<B>> {
+        let size = match options.viewport {
+            Viewport::Fullscreen | Viewport::Inline(_) => backend.size()?,
+            Viewport::Fixed(area) => area,
+        };
+        let (viewport_area, cursor_pos) = match options.viewport {
+            Viewport::Fullscreen => (size, (0, 0)),
+            Viewport::Inline(height) => compute_inline_size(&mut backend, height, size, 0)?,
+            Viewport::Fixed(area) => (area, (area.left(), area.top())),
+        };
         Ok(Terminal {
             backend,
-            buffers: [
-                Buffer::empty(options.viewport.area),
-                Buffer::empty(options.viewport.area),
-            ],
+            buffers: [Buffer::empty(viewport_area), Buffer::empty(viewport_area)],
             current: 0,
             hidden_cursor: false,
             viewport: options.viewport,
+            viewport_area,
+            last_known_size: size,
+            last_known_cursor_pos: cursor_pos,
         })
     }
 
@@ -225,24 +218,46 @@ where
         let previous_buffer = &self.buffers[1 - self.current];
         let current_buffer = &self.buffers[self.current];
         let updates = previous_buffer.diff(current_buffer);
+        if let Some((col, row, _)) = updates.last() {
+            self.last_known_cursor_pos = (*col, *row);
+        }
         self.backend.draw(updates.into_iter())
     }
 
     /// Updates the Terminal so that internal buffers match the requested size. Requested size will
     /// be saved so the size can remain consistent when rendering.
     /// This leads to a full clear of the screen.
-    pub fn resize(&mut self, area: Rect) -> io::Result<()> {
+    pub fn resize(&mut self, size: Rect) -> io::Result<()> {
+        let next_area = match self.viewport {
+            Viewport::Fullscreen => size,
+            Viewport::Inline(height) => {
+                let offset_in_previous_viewport = self
+                    .last_known_cursor_pos
+                    .1
+                    .saturating_sub(self.viewport_area.top());
+                compute_inline_size(&mut self.backend, height, size, offset_in_previous_viewport)?.0
+            }
+            Viewport::Fixed(area) => area,
+        };
+        self.set_viewport_area(next_area);
+        self.clear()?;
+
+        self.last_known_size = size;
+        Ok(())
+    }
+
+    fn set_viewport_area(&mut self, area: Rect) {
         self.buffers[self.current].resize(area);
         self.buffers[1 - self.current].resize(area);
-        self.viewport.area = area;
-        self.clear()
+        self.viewport_area = area;
     }
 
     /// Queries the backend for size and resizes if it doesn't match the previous size.
     pub fn autoresize(&mut self) -> io::Result<()> {
-        if self.viewport.resize_behavior == ResizeBehavior::Auto {
+        // fixed viewports do not get autoresized
+        if matches!(self.viewport, Viewport::Fullscreen | Viewport::Inline(_)) {
             let size = self.size()?;
-            if size != self.viewport.area {
+            if size != self.last_known_size {
                 self.resize(size)?;
             }
         };
@@ -283,9 +298,10 @@ where
 
         // Flush
         self.backend.flush()?;
+
         Ok(CompletedFrame {
             buffer: &self.buffers[1 - self.current],
-            area: self.viewport.area,
+            area: self.last_known_size,
         })
     }
 
@@ -306,12 +322,27 @@ where
     }
 
     pub fn set_cursor(&mut self, x: u16, y: u16) -> io::Result<()> {
-        self.backend.set_cursor(x, y)
+        self.backend.set_cursor(x, y)?;
+        self.last_known_cursor_pos = (x, y);
+        Ok(())
     }
 
     /// Clear the terminal and force a full redraw on the next draw call.
     pub fn clear(&mut self) -> io::Result<()> {
-        self.backend.clear()?;
+        match self.viewport {
+            Viewport::Fullscreen => self.backend.clear_region(ClearType::All)?,
+            Viewport::Inline(_) => {
+                self.backend
+                    .set_cursor(self.viewport_area.left(), self.viewport_area.top())?;
+                self.backend.clear_region(ClearType::AfterCursor)?;
+            }
+            Viewport::Fixed(area) => {
+                for row in area.top()..area.bottom() {
+                    self.backend.set_cursor(0, row)?;
+                    self.backend.clear_region(ClearType::AfterCursor)?;
+                }
+            }
+        }
         // Reset the back buffer to make sure the next update will redraw everything.
         self.buffers[1 - self.current].reset();
         Ok(())
@@ -321,4 +352,130 @@ where
     pub fn size(&self) -> io::Result<Rect> {
         self.backend.size()
     }
+
+    /// Insert some content before the current inline viewport. This has no effect when the
+    /// viewport is fullscreen.
+    ///
+    /// This function scrolls down the current viewport by the given height. The newly freed space is
+    /// then made available to the `draw_fn` closure through a writable `Buffer`.
+    ///
+    /// Before:
+    /// ```ignore
+    /// +-------------------+
+    /// |                   |
+    /// |      viewport     |
+    /// |                   |
+    /// +-------------------+
+    /// ```
+    ///
+    /// After:
+    /// ```ignore
+    /// +-------------------+
+    /// |      buffer       |
+    /// +-------------------+
+    /// +-------------------+
+    /// |                   |
+    /// |      viewport     |
+    /// |                   |
+    /// +-------------------+
+    /// ```
+    ///
+    /// # Examples
+    ///
+    /// ## Insert a single line before the current viewport
+    ///
+    /// ```rust
+    /// # use ratatui::widgets::{Paragraph, Widget};
+    /// # use ratatui::text::{Spans, Span};
+    /// # use ratatui::style::{Color, Style};
+    /// # use ratatui::{Terminal};
+    /// # use ratatui::backend::TestBackend;
+    /// # let backend = TestBackend::new(10, 10);
+    /// # let mut terminal = Terminal::new(backend).unwrap();
+    /// terminal.insert_before(1, |buf| {
+    ///     Paragraph::new(Spans::from(vec![
+    ///         Span::raw("This line will be added "),
+    ///         Span::styled("before", Style::default().fg(Color::Blue)),
+    ///         Span::raw(" the current viewport")
+    ///     ])).render(buf.area, buf);
+    /// });
+    /// ```
+    pub fn insert_before<F>(&mut self, height: u16, draw_fn: F) -> io::Result<()>
+    where
+        F: FnOnce(&mut Buffer),
+    {
+        if !matches!(self.viewport, Viewport::Inline(_)) {
+            return Ok(());
+        }
+
+        self.clear()?;
+        let height = height.min(self.last_known_size.height);
+        self.backend.append_lines(height)?;
+        let missing_lines =
+            height.saturating_sub(self.last_known_size.bottom() - self.viewport_area.top());
+        let area = Rect {
+            x: self.viewport_area.left(),
+            y: self.viewport_area.top().saturating_sub(missing_lines),
+            width: self.viewport_area.width,
+            height,
+        };
+        let mut buffer = Buffer::empty(area);
+
+        draw_fn(&mut buffer);
+
+        let iter = buffer.content.iter().enumerate().map(|(i, c)| {
+            let (x, y) = buffer.pos_of(i);
+            (x, y, c)
+        });
+        self.backend.draw(iter)?;
+        self.backend.flush()?;
+
+        let remaining_lines = self.last_known_size.height - area.bottom();
+        let missing_lines = self.viewport_area.height.saturating_sub(remaining_lines);
+        self.backend.append_lines(self.viewport_area.height)?;
+
+        self.set_viewport_area(Rect {
+            x: area.left(),
+            y: area.bottom().saturating_sub(missing_lines),
+            width: area.width,
+            height: self.viewport_area.height,
+        });
+
+        Ok(())
+    }
+}
+
+fn compute_inline_size<B: Backend>(
+    backend: &mut B,
+    height: u16,
+    size: Rect,
+    offset_in_previous_viewport: u16,
+) -> io::Result<(Rect, (u16, u16))> {
+    let pos = backend.get_cursor()?;
+    let mut row = pos.1;
+
+    let max_height = size.height.min(height);
+
+    let lines_after_cursor = height
+        .saturating_sub(offset_in_previous_viewport)
+        .saturating_sub(1);
+
+    backend.append_lines(lines_after_cursor)?;
+
+    let available_lines = size.height.saturating_sub(row).saturating_sub(1);
+    let missing_lines = lines_after_cursor.saturating_sub(available_lines);
+    if missing_lines > 0 {
+        row = row.saturating_sub(missing_lines);
+    }
+    row = row.saturating_sub(offset_in_previous_viewport);
+
+    Ok((
+        Rect {
+            x: 0,
+            y: row,
+            width: size.width,
+            height: max_height,
+        },
+        pos,
+    ))
 }

tests/backend_termion.rs

@@ -15,7 +15,7 @@ fn backend_termion_should_only_write_diffs() -> Result<(), Box<dyn std::error::E
         let mut terminal = Terminal::with_options(
             backend,
             TerminalOptions {
-                viewport: Viewport::fixed(area),
+                viewport: Viewport::Fixed(area),
             },
         )?;
         terminal.draw(|f| {