Prevent partial widget being written to prerender cache

src/components/backlog.rs

@@ -204,9 +204,12 @@ impl Backlog {
               key: text_area.width,
               value: PrerenderValue::Rendered(buf),
             });
-          }
 
-          height
+            height
+          } else {
+            // the widget overflowed and we drew on the whole area
+            text_area.height
+          }
         },
       };
 

src/widgets/bottom_aligned_paragraph.rs

@@ -89,6 +89,7 @@ impl<'a> OverlappableWidget for BottomAlignedParagraph<'a> {
 
     let text_area = area; // Borders not supported by BottomAlignedParagraph (yet?)
     let text_area_height = text_area.height as usize;
+    let actual_height = lines.len();
     let lines = if lines.len() > text_area_height {
       // Overflow; keep only the last lines
       &lines[..text_area_height]
@@ -119,6 +120,6 @@ impl<'a> OverlappableWidget for BottomAlignedParagraph<'a> {
       }
     }
 
-    lines.len() as u16
+    actual_height as u16
   }
 }

tests/components/backlog.rs

@@ -342,3 +342,140 @@ fn test_cache_moved() {
   ]);
   assert_eq!(buf, expected);
 }
+
+#[test]
+fn test_overflow_and_scroll() {
+  let mut bl = Backlog::default();
+  let prerender1 = Prerender::new();
+  let prerender2 = Prerender::new();
+  let prerender3 = Prerender::new();
+
+  let item1 = BufferItem {
+    text: Text::raw("line1 x"),
+    prerender: &prerender1,
+  };
+  let item2 = BufferItem {
+    text: Text::raw("line2  y\nline3  y\nline4  y"),
+    prerender: &prerender2,
+  };
+  let item3 = BufferItem {
+    text: Text::raw("line5   z"),
+    prerender: &prerender3,
+  };
+  let mut buf = Buffer::empty(rect(0, 0, 14, 7));
+  bl.draw_items(
+    &mut buf,
+    rect(1, 1, 12, 5),
+    vec![item3, item2, item1].into_iter(),
+  )
+  .context("Failed to draw")
+  .unwrap();
+  let expected = Buffer::with_lines(vec![
+    "              ",
+    " ┌──────────┐ ",
+    " │line3  y  │ ",
+    " │line4  y  │ ",
+    " │line5   z │ ",
+    " └──────────┘ ",
+    "              ",
+  ]);
+  assert_eq!(buf, expected);
+
+  bl.scroll_up(1);
+
+  let item1 = BufferItem {
+    text: Text::raw("line1 x"),
+    prerender: &prerender1,
+  };
+  let item2 = BufferItem {
+    text: Text::raw("line2  y\nline3  y\nline4  y"),
+    prerender: &prerender2,
+  };
+  let item3 = BufferItem {
+    text: Text::raw("line5   z"),
+    prerender: &prerender3,
+  };
+  let mut buf = Buffer::empty(rect(0, 0, 14, 7));
+  bl.draw_items(
+    &mut buf,
+    rect(1, 1, 12, 5),
+    vec![item3, item2, item1].into_iter(),
+  )
+  .context("Failed to draw")
+  .unwrap();
+  let expected = Buffer::with_lines(vec![
+    "              ",
+    " ┌──────────┐ ",
+    " │line2  y  │ ",
+    " │line3  y  │ ",
+    " │line4  y  │ ",
+    " └──────────┘ ",
+    "              ",
+  ]);
+  assert_eq!(buf, expected);
+
+  bl.scroll_up(1);
+
+  let item1 = BufferItem {
+    text: Text::raw("line1 x"),
+    prerender: &prerender1,
+  };
+  let item2 = BufferItem {
+    text: Text::raw("line2  y\nline3  y\nline4  y"),
+    prerender: &prerender2,
+  };
+  let item3 = BufferItem {
+    text: Text::raw("line5   z"),
+    prerender: &prerender3,
+  };
+  let mut buf = Buffer::empty(rect(0, 0, 14, 7));
+  bl.draw_items(
+    &mut buf,
+    rect(1, 1, 12, 5),
+    vec![item3, item2, item1].into_iter(),
+  )
+  .context("Failed to draw")
+  .unwrap();
+  let expected = Buffer::with_lines(vec![
+    "              ",
+    " ┌──────────┐ ",
+    " │line1 x   │ ",
+    " │line2  y  │ ",
+    " │line3  y  │ ",
+    " └──────────┘ ",
+    "              ",
+  ]);
+  assert_eq!(buf, expected);
+  bl.scroll_up(1);
+
+  let item1 = BufferItem {
+    text: Text::raw("line1 x"),
+    prerender: &prerender1,
+  };
+  let item2 = BufferItem {
+    text: Text::raw("line2  y\nline3  y\nline4  y"),
+    prerender: &prerender2,
+  };
+  let item3 = BufferItem {
+    text: Text::raw("line5   z"),
+    prerender: &prerender3,
+  };
+  let mut buf = Buffer::empty(rect(0, 0, 14, 7));
+  bl.draw_items(
+    &mut buf,
+    rect(1, 1, 12, 5),
+    vec![item3, item2, item1].into_iter(),
+  )
+  .context("Failed to draw")
+  .unwrap();
+  let expected = Buffer::with_lines(vec![
+    "              ",
+    " ┌──────────┐ ",
+    " │          │ ",
+    " │line1 x   │ ",
+    " │line2  y  │ ",
+    " └──────────┘ ",
+    "              ",
+  ]);
+  assert_eq!(buf, expected);
+}