Keep viewport in place during resize

Fixes #4879.

Co-authored-by: Christian Duerr <contact@christianduerr.com>

CHANGELOG.md

@@ -24,6 +24,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Incorrect vi cursor position after leaving search
 - Clicking on URLs on Windows incorrectly opens File Explorer
 - Incorrect underline cursor thickness on wide cell
+- Viewport moving around when resizing while scrolled into history
 
 ### Removed
 

alacritty_terminal/src/grid/mod.rs

@@ -502,7 +502,7 @@ pub trait Dimensions {
     /// Number of invisible lines part of the scrollback history.
     #[inline]
     fn history_size(&self) -> usize {
-        self.total_lines() - self.screen_lines()
+        self.total_lines().saturating_sub(self.screen_lines())
     }
 }
 

alacritty_terminal/src/grid/resize.rs

@@ -187,7 +187,7 @@ impl<T: GridCell + Default + PartialEq + Clone> Grid<T> {
 
                 cursor_line_delta += line_delta.0 as usize;
             } else if row.is_clear() {
-                if i + reversed.len() >= self.lines {
+                if i <= self.display_offset {
                     // Since we removed a line, rotate down the viewport.
                     self.display_offset = self.display_offset.saturating_sub(1);
                 }
@@ -354,6 +354,11 @@ impl<T: GridCell + Default + PartialEq + Clone> Grid<T> {
                         wrapped.resize_with(columns, T::default);
                     }
                     row = Row::from_vec(wrapped, occ);
+
+                    if i <= self.display_offset {
+                        // Since we added a new line, rotate up the viewport.
+                        self.display_offset += 1;
+                    }
                 }
             }
         }