performance(terminal): improvements to reflow performance by removing O(n^2) behavior (zellij-org#3045)

* refactor: Simplify transfer_rows_from_viewport_to_lines_above

next_lines is always consolidated to a single Row, which immediately
gets removed - we can remove some dead code as a result

* perf: Batch remove rows from the viewport for performance

Given a 1MB line catted into the terminal, a toggle-fullscreen +
toggle-fullscreen + close-pane + `run true` goes from ~9s to ~3s

* perf: Optimize Row::drain_until by splitting chars in one step

Given a 10MB line catted into the terminal, a toggle-fullscreen +
toggle-fullscreen + close-pane + `run true` goes from ~23s to ~20s

Author: aidanhs

zellij-server/src/panes/grid.rs

@@ -166,37 +166,24 @@ fn transfer_rows_from_viewport_to_lines_above(
     count: usize,
     max_viewport_width: usize,
 ) -> isize {
-    let mut next_lines: Vec<Row> = vec![];
     let mut transferred_rows_count: isize = 0;
-    for _ in 0..count {
-        if next_lines.is_empty() {
-            if !viewport.is_empty() {
-                let next_line = viewport.remove(0);
-                transferred_rows_count +=
-                    calculate_row_display_height(next_line.width(), max_viewport_width) as isize;
-                if !next_line.is_canonical {
-                    let mut bottom_canonical_row_and_wraps_in_dst =
-                        get_lines_above_bottom_canonical_row_and_wraps(lines_above);
-                    next_lines.append(&mut bottom_canonical_row_and_wraps_in_dst);
-                }
-                next_lines.push(next_line);
-                next_lines = vec![Row::from_rows(next_lines)];
-            } else {
-                break; // no more rows
-            }
-        }
-        let dropped_line_width = bounded_push(lines_above, sixel_grid, next_lines.remove(0));
+    let drained_lines = std::cmp::min(count, viewport.len());
+    for next_line in viewport.drain(..drained_lines) {
+        let mut next_lines: Vec<Row> = vec![];
+        transferred_rows_count +=
+            calculate_row_display_height(next_line.width(), max_viewport_width) as isize;
+        if !next_line.is_canonical {
+            let mut bottom_canonical_row_and_wraps_in_dst =
+                get_lines_above_bottom_canonical_row_and_wraps(lines_above);
+            next_lines.append(&mut bottom_canonical_row_and_wraps_in_dst);
+        }
+        next_lines.push(next_line);
+        let dropped_line_width = bounded_push(lines_above, sixel_grid, Row::from_rows(next_lines));
         if let Some(width) = dropped_line_width {
             transferred_rows_count -=
                 calculate_row_display_height(width, max_viewport_width) as isize;
         }
     }
-    if !next_lines.is_empty() {
-        let excess_rows = Row::from_rows(next_lines).split_to_rows_of_length(max_viewport_width);
-        for row in excess_rows {
-            viewport.insert(0, row);
-        }
-    }
     transferred_rows_count
 }
 
@@ -3379,19 +3366,20 @@ impl Row {
         self.width = None;
     }
     pub fn drain_until(&mut self, x: usize) -> VecDeque<TerminalCharacter> {
-        let mut drained_part: VecDeque<TerminalCharacter> = VecDeque::new();
         let mut drained_part_len = 0;
-        while let Some(next_character) = self.columns.remove(0) {
+        let mut split_pos = 0;
+        for next_character in self.columns.iter() {
             // drained_part_len == 0 here is so that if the grid is resized
             // to a size of 1, we won't drop wide characters
             if drained_part_len + next_character.width <= x || drained_part_len == 0 {
-                drained_part.push_back(next_character);
                 drained_part_len += next_character.width;
+                split_pos += 1
             } else {
-                self.columns.push_front(next_character); // put it back
                 break;
             }
         }
+        // Can't use split_off because it doesn't reduce capacity, causing OOM with long lines
+        let drained_part = self.columns.drain(..split_pos).collect();
         self.width = None;
         drained_part
     }