fix: scale thumbnails to fill the preview window

src/thumbnail.rs

@@ -95,7 +95,7 @@ impl Thumbnail {
 
         let mut message = self.error.as_deref().unwrap_or("Loading thumbnail...");
         if let Some(image) = self.image.as_mut() {
-            let resize = Resize::Fit(Some(FilterType::Triangle));
+            let resize = Resize::Scale(Some(FilterType::Triangle));
             if let Some(size) = image.protocol.size_for(resize.clone(), area.into()) {
                 frame.render_stateful_widget(
                     StatefulImage::new().resize(resize),
@@ -274,6 +274,66 @@ mod tests {
         assert!(thumbnail.error.is_none());
     }
 
+    #[test]
+    fn small_thumbnail_fills_popup_proportionally_and_resizes() {
+        use ratatui::{Terminal, backend::TestBackend, style::Color};
+        use std::time::Duration;
+
+        let mut thumbnail = Thumbnail::new();
+        let font = thumbnail.picker.font_size();
+        let image = image::DynamicImage::ImageRgb8(image::RgbImage::from_pixel(
+            48,
+            36,
+            image::Rgb([240, 40, 70]),
+        ));
+        let (requests_tx, requests_rx) = mpsc::channel::<ResizeRequest>();
+        let (replies_tx, replies_rx) = mpsc::channel();
+        thumbnail.selected = Some("thumbnail".into());
+        thumbnail.image = Some(ThumbnailImage {
+            id: "thumbnail".into(),
+            protocol: ThreadProtocol::new(
+                requests_tx,
+                Some(thumbnail.picker.new_resize_protocol(image)),
+            ),
+            replies: replies_rx,
+            complete: false,
+        });
+        let mut terminal = Terminal::new(TestBackend::new(100, 40)).unwrap();
+        for area in [Rect::new(3, 2, 80, 18), Rect::new(7, 4, 30, 8)] {
+            terminal.draw(|frame| thumbnail.draw(frame, area)).unwrap();
+            let response = requests_rx
+                .recv_timeout(Duration::from_secs(2))
+                .unwrap()
+                .resize_encode()
+                .unwrap();
+            replies_tx.send(Ok(response)).unwrap();
+            terminal.draw(|frame| thumbnail.draw(frame, area)).unwrap();
+            let buffer = terminal.backend().buffer();
+            let pixels: Vec<_> = (0..40)
+                .flat_map(|y| (0..100).map(move |x| (x, y)))
+                .filter(|&(x, y)| {
+                    let cell = &buffer[(x, y)];
+                    matches!(cell.fg, Color::Rgb(..)) || matches!(cell.bg, Color::Rgb(..))
+                })
+                .collect();
+            let left = pixels.iter().map(|&(x, _)| x).min().unwrap();
+            let right = pixels.iter().map(|&(x, _)| x).max().unwrap() + 1;
+            let top = pixels.iter().map(|&(_, y)| y).min().unwrap();
+            let bottom = pixels.iter().map(|&(_, y)| y).max().unwrap() + 1;
+            assert!(left >= area.x && right <= area.right());
+            assert!(top >= area.y && bottom <= area.bottom());
+            assert!(right - left == area.width || bottom - top == area.height);
+            assert!((left - area.x).abs_diff(area.right() - right) <= 1);
+            assert!((top - area.y).abs_diff(area.bottom() - bottom) <= 1);
+            let width = u32::from(right - left) * u32::from(font.width);
+            let height = u32::from(bottom - top) * u32::from(font.height);
+            assert!(
+                width.abs_diff(height * 4 / 3)
+                    <= u32::from(font.width) + u32::from(font.height) * 4 / 3
+            );
+        }
+    }
+
     #[test]
     fn resized_image_preserves_aspect_and_stays_inside_popup() {
         let picker = Picker::halfblocks();