performance: de-bottleneck resizing/line-wrapping (#4778)

* vec -> vecdeque

* use cached width when resizing

* reuse optimized split_to_rows_of_length function

* accept vecdeque directly - remove unsafe

* add pr

5 files changed, 70 insertions, 77 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 52b6402e..4d711249 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -64,6 +64,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * feat: allow specifying "close_on_exit" when opening panes in place from keybinding or cli (https://github.com/zellij-org/zellij/pull/4760)
 * fix: respect cursor blink and shape in the web client even when terminal attempts to override it (https://github.com/zellij-org/zellij/pull/4762)
 * feat: allow cli/plugins/keybindings to atomically show/hide floating panes rather than just toggle (https://github.com/zellij-org/zellij/pull/4764)
+* fix: improve line-wrapping/resize performance (https://github.com/zellij-org/zellij/pull/4778)
 
 
 ## [0.43.1] - 2025-08-08
diff --git a/zellij-server/src/panes/grid.rs b/zellij-server/src/panes/grid.rs
index d7cb3576..04563b2a 100644
--- a/zellij-server/src/panes/grid.rs
+++ b/zellij-server/src/panes/grid.rs
@@ -41,7 +41,7 @@ use crate::panes::Selection;
 use crate::ui::components::UiComponentParser;
 use zellij_utils::data::PaneContents;
 
-fn get_top_non_canonical_rows(rows: &mut Vec<Row>) -> Vec<Row> {
+fn get_top_non_canonical_rows(rows: &mut VecDeque<Row>) -> Vec<Row> {
     let mut index_of_last_non_canonical_row = None;
     for (i, row) in rows.iter().enumerate() {
         if row.is_canonical {
@@ -74,7 +74,7 @@ fn get_lines_above_bottom_canonical_row_and_wraps(rows: &mut VecDeque<Row>) -> V
     }
 }
 
-fn get_viewport_bottom_canonical_row_and_wraps(viewport: &mut Vec<Row>) -> Vec<Row> {
+fn get_viewport_bottom_canonical_row_and_wraps(viewport: &mut VecDeque<Row>) -> Vec<Row> {
     let mut index_of_last_non_canonical_row = None;
     for (i, row) in viewport.iter().enumerate().rev() {
         index_of_last_non_canonical_row = Some(i);
@@ -90,7 +90,7 @@ fn get_viewport_bottom_canonical_row_and_wraps(viewport: &mut Vec<Row>) -> Vec<R
     }
 }
 
-fn get_top_canonical_row_and_wraps(rows: &mut Vec<Row>) -> Vec<Row> {
+fn get_top_canonical_row_and_wraps(rows: &mut VecDeque<Row>) -> Vec<Row> {
     let mut index_of_first_non_canonical_row = None;
     let mut end_index_of_first_canonical_line = None;
     for (i, row) in rows.iter().enumerate() {
@@ -119,7 +119,7 @@ fn get_top_canonical_row_and_wraps(rows: &mut Vec<Row>) -> Vec<Row> {
 
 fn transfer_rows_from_lines_above_to_viewport(
     lines_above: &mut VecDeque<Row>,
-    viewport: &mut Vec<Row>,
+    viewport: &mut VecDeque<Row>,
     sixel_grid: &mut SixelGrid,
     count: usize,
     max_viewport_width: usize,
@@ -147,7 +147,7 @@ fn transfer_rows_from_lines_above_to_viewport(
                 None => break, // no more rows
             }
         }
-        viewport.insert(0, next_lines.pop().unwrap());
+        viewport.push_front(next_lines.pop().unwrap());
         lines_added_to_viewport += 1;
     }
     if !next_lines.is_empty() {
@@ -161,7 +161,7 @@ fn transfer_rows_from_lines_above_to_viewport(
 }
 
 fn transfer_rows_from_viewport_to_lines_above(
-    viewport: &mut Vec<Row>,
+    viewport: &mut VecDeque<Row>,
     lines_above: &mut VecDeque<Row>,
     sixel_grid: &mut SixelGrid,
     count: usize,
@@ -189,8 +189,8 @@ fn transfer_rows_from_viewport_to_lines_above(
 }
 
 fn transfer_rows_from_lines_below_to_viewport(
-    lines_below: &mut Vec<Row>,
-    viewport: &mut Vec<Row>,
+    lines_below: &mut VecDeque<Row>,
+    viewport: &mut VecDeque<Row>,
     count: usize,
     max_viewport_width: usize,
 ) {
@@ -218,13 +218,13 @@ fn transfer_rows_from_lines_below_to_viewport(
         }
         for _ in 0..(lines_pulled_from_viewport + 1) {
             if !next_lines.is_empty() {
-                viewport.push(next_lines.remove(0));
+                viewport.push_back(next_lines.remove(0));
             }
         }
     }
     if !next_lines.is_empty() {
         let excess_row = Row::from_rows(next_lines);
-        lines_below.insert(0, excess_row);
+        lines_below.push_front(excess_row);
     }
 }
 
@@ -352,8 +352,8 @@ fn utf8_mouse_coordinates(column: usize, line: isize) -> Vec<u8> {
 #[derive(Clone)]
 pub struct Grid {
     pub(crate) lines_above: VecDeque<Row>,
-    pub(crate) viewport: Vec<Row>,
-    pub(crate) lines_below: Vec<Row>,
+    pub(crate) viewport: VecDeque<Row>,
+    pub(crate) lines_below: VecDeque<Row>,
     horizontal_tabstops: BTreeSet<usize>,
     alternate_screen_state: Option<AlternateScreenState>,
     cursor: Cursor,
@@ -474,7 +474,7 @@ impl Default for MouseTracking {
 
 impl Debug for Grid {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        let mut buffer: Vec<Row> = self.viewport.clone();
+        let mut buffer: Vec<Row> = Vec::from(self.viewport.clone());
         // pad buffer
         for _ in buffer.len()..self.height {
             buffer.push(Row::new().canonical());
@@ -543,8 +543,8 @@ impl Grid {
         let _ = SCROLL_BUFFER_SIZE.set(DEFAULT_SCROLL_BUFFER_SIZE);
         Grid {
             lines_above: VecDeque::new(),
-            viewport: vec![Row::new().canonical()],
-            lines_below: vec![],
+            viewport: VecDeque::from(vec![Row::new().canonical()]),
+            lines_below: VecDeque::new(),
             horizontal_tabstops: create_horizontal_tabstops(columns),
             cursor: Cursor::new(0, 0, styled_underlines),
             cursor_is_hidden: false,
@@ -648,10 +648,10 @@ impl Grid {
         )
     }
 
-    fn recalculate_scrollback_buffer_count(&self) -> usize {
+    fn recalculate_scrollback_buffer_count(&mut self) -> usize {
         let mut scrollback_buffer_count = 0;
-        for row in &self.lines_above {
-            let row_width = row.width();
+        for row in &mut self.lines_above {
+            let row_width = row.width_cached();
             // rows in lines_above are unwrapped, so we need to account for that
             if row_width > self.width {
                 scrollback_buffer_count += calculate_row_display_height(row_width, self.width);
@@ -754,8 +754,8 @@ impl Grid {
         let mut found_something = false;
         if !self.lines_above.is_empty() && self.viewport.len() == self.height {
             self.is_scrolled = true;
-            let line_to_push_down = self.viewport.pop().unwrap();
-            self.lines_below.insert(0, line_to_push_down);
+            let line_to_push_down = self.viewport.pop_back().unwrap();
+            self.lines_below.push_front(line_to_push_down);
 
             let transferred_rows_height = transfer_rows_from_lines_above_to_viewport(
                 &mut self.lines_above,
@@ -783,7 +783,7 @@ impl Grid {
             && self.viewport.len() == self.height
             && !self.viewport.is_empty()
         {
-            let mut line_to_push_up = self.viewport.remove(0);
+            let mut line_to_push_up = self.viewport.pop_front().unwrap();
 
             self.scrollback_buffer_lines +=
                 calculate_row_display_height(line_to_push_up.width(), self.width);
@@ -927,32 +927,16 @@ impl Grid {
 
             let mut new_viewport_rows = vec![];
             for mut canonical_line in viewport_canonical_lines {
-                let mut canonical_line_parts: Vec<Row> = vec![];
-                if canonical_line.columns.is_empty() {
-                    canonical_line_parts.push(Row::new().canonical());
-                }
-                while !canonical_line.columns.is_empty() {
-                    let next_wrap = canonical_line.drain_until(new_columns);
-                    // If the next character is wider than the grid (i.e. there is nothing in
-                    // `next_wrap`, then just abort the resizing
-                    if next_wrap.is_empty() {
-                        break;
-                    }
-                    let row = Row::from_columns(next_wrap);
-                    // if there are no more parts, this row is canonical as long as it originally
-                    // was canonical (it might not have been for example if it's the first row in
-                    // the viewport, and the actual canonical row is above it in the scrollback)
-                    let row = if canonical_line_parts.is_empty() && canonical_line.is_canonical {
-                        row.canonical()
-                    } else {
-                        row
-                    };
-                    canonical_line_parts.push(row);
+                let mut canonical_line_parts = canonical_line.split_to_rows_of_length(new_columns);
+                // If a character is wider than the grid, split_to_rows_of_length returns an empty
+                // vec — skip the line, matching the old `break` behavior
+                if canonical_line_parts.is_empty() {
+                    continue;
                 }
                 new_viewport_rows.append(&mut canonical_line_parts);
             }
 
-            self.viewport = new_viewport_rows;
+            self.viewport = VecDeque::from(new_viewport_rows);
 
             let mut new_cursor_y = self.canonical_line_y_coordinates(cursor_canonical_line_index)
                 + (cursor_index_in_canonical_line / new_columns);
@@ -1124,7 +1108,7 @@ impl Grid {
         y_offset: usize,
     ) -> (Vec<CharacterChunk>, Vec<SixelImageChunk>) {
         let changed_character_chunks = self.output_buffer.changed_chunks_in_viewport(
-            &self.viewport,
+            self.viewport.make_contiguous(),
             self.width,
             self.height,
             x_offset,
@@ -1168,10 +1152,12 @@ impl Grid {
                     .serialize(to_serialize.as_slice(), self.osc8_hyperlinks, None)
                     .ok()
             },
-            None => self
-                .output_buffer
-                .serialize(&self.viewport, self.osc8_hyperlinks, None)
-                .ok(),
+            None => {
+                let viewport_vec: Vec<Row> = self.viewport.iter().cloned().collect();
+                self.output_buffer
+                    .serialize(&viewport_vec, self.osc8_hyperlinks, None)
+                    .ok()
+            },
         }
     }
     pub fn render(
@@ -1363,7 +1349,8 @@ impl Grid {
 
         for _ in 0..self.height {
             let columns = VecDeque::from(vec![character.clone(); self.width]);
-            self.viewport.push(Row::from_columns(columns).canonical());
+            self.viewport
+                .push_back(Row::from_columns(columns).canonical());
         }
         self.output_buffer.update_all_lines();
     }
@@ -1391,10 +1378,10 @@ impl Grid {
                 if self.alternate_screen_state.is_none() {
                     self.transfer_rows_to_lines_above(1);
                 } else if !self.viewport.is_empty() {
-                    self.viewport.remove(0);
+                    self.viewport.pop_front();
                 }
 
-                self.viewport.push(Row::new().canonical());
+                self.viewport.push_back(Row::new().canonical());
                 self.selection.move_up(1);
             } else {
                 if scroll_region_top < self.viewport.len() {
@@ -1404,7 +1391,7 @@ impl Grid {
                     self.viewport
                         .insert(scroll_region_bottom, Row::new().canonical());
                 } else {
-                    self.viewport.push(Row::new().canonical());
+                    self.viewport.push_back(Row::new().canonical());
                 }
             }
             self.output_buffer.update_all_lines(); // TODO: only update scroll region lines
@@ -1415,7 +1402,7 @@ impl Grid {
             // but for some reason this breaks rendering in various situations
             // it needs to be investigated and fixed
             let new_row = Row::new().canonical();
-            self.viewport.push(new_row);
+            self.viewport.push_back(new_row);
         }
         if self.cursor.y == self.height.saturating_sub(1) {
             self.output_buffer.update_all_lines();
@@ -1477,10 +1464,10 @@ impl Grid {
             None => {
                 // pad lines until cursor if they do not exist
                 for _ in self.viewport.len()..self.cursor.y {
-                    self.viewport.push(Row::new().canonical());
+                    self.viewport.push_back(Row::new().canonical());
                 }
                 self.viewport
-                    .push(Row::new().with_character(terminal_character).canonical());
+                    .push_back(Row::new().with_character(terminal_character).canonical());
                 self.output_buffer.update_line(self.cursor.y);
             },
         }
@@ -1570,17 +1557,17 @@ impl Grid {
                 self.transfer_rows_to_lines_above(1);
                 self.hyperlink_tracker.offset_cursor_lines(1);
             } else if !self.viewport.is_empty() {
-                self.viewport.remove(0);
+                self.viewport.pop_front();
             }
             let wrapped_row = Row::new();
-            self.viewport.push(wrapped_row);
+            self.viewport.push_back(wrapped_row);
             self.selection.move_up(1);
             self.output_buffer.update_all_lines();
         } else {
             self.cursor.y += 1;
             if self.viewport.len() <= self.cursor.y {
                 let line_wrapped_row = Row::new();
-                self.viewport.push(line_wrapped_row);
+                self.viewport.push_back(line_wrapped_row);
                 self.output_buffer.update_line(self.cursor.y);
             } else if let Some(current_line) = self.viewport.get_mut(self.cursor.y) {
                 current_line.is_canonical = false;
@@ -1606,7 +1593,8 @@ impl Grid {
     fn pad_lines_until(&mut self, position: usize, pad_character: TerminalCharacter) {
         for _ in self.viewport.len()..=position {
             let columns = VecDeque::from(vec![pad_character.clone(); self.width]);
-            self.viewport.push(Row::from_columns(columns).canonical());
+            self.viewport
+                .push_back(Row::from_columns(columns).canonical());
             self.output_buffer.update_line(self.viewport.len() - 1);
         }
     }
@@ -1720,7 +1708,8 @@ impl Grid {
                     self.viewport
                         .insert(scroll_region_bottom, Row::from_columns(columns).canonical());
                 } else {
-                    self.viewport.push(Row::from_columns(columns).canonical());
+                    self.viewport
+                        .push_back(Row::from_columns(columns).canonical());
                 }
             }
             self.output_buffer.update_all_lines(); // TODO: move accurately
@@ -1806,9 +1795,9 @@ impl Grid {
     }
     pub fn reset_terminal_state(&mut self) {
         self.lines_above = VecDeque::new();
-        self.lines_below = vec![];
+        self.lines_below = VecDeque::new();
         self.is_scrolled = false;
-        self.viewport = vec![Row::new().canonical()];
+        self.viewport = VecDeque::from(vec![Row::new().canonical()]);
         self.alternate_screen_state = None;
         self.cursor_key_mode = false;
         self.clear_viewport_before_rendering = true;
@@ -3048,8 +3037,10 @@ impl Perform for Grid {
                             // enter alternate buffer
                             let current_lines_above =
                                 std::mem::replace(&mut self.lines_above, VecDeque::new());
-                            let current_viewport =
-                                std::mem::replace(&mut self.viewport, vec![Row::new().canonical()]);
+                            let current_viewport = std::mem::replace(
+                                &mut self.viewport,
+                                VecDeque::from(vec![Row::new().canonical()]),
+                            );
                             let current_cursor = std::mem::replace(
                                 &mut self.cursor,
                                 Cursor::new(0, 0, self.styled_underlines),
@@ -3543,7 +3534,7 @@ impl Perform for Grid {
 #[derive(Clone)]
 pub struct AlternateScreenState {
     lines_above: VecDeque<Row>,
-    viewport: Vec<Row>,
+    viewport: VecDeque<Row>,
     cursor: Cursor,
     sixel_grid: SixelGrid,
     supports_kitty_keyboard_protocol: bool,
@@ -3551,7 +3542,7 @@ pub struct AlternateScreenState {
 impl AlternateScreenState {
     pub fn new(
         lines_above: VecDeque<Row>,
-        viewport: Vec<Row>,
+        viewport: VecDeque<Row>,
         cursor: Cursor,
         sixel_grid: SixelGrid,
         supports_kitty_keyboard_protocol: bool,
@@ -3567,7 +3558,7 @@ impl AlternateScreenState {
     pub fn apply_contents_to(
         &mut self,
         lines_above: &mut VecDeque<Row>,
-        viewport: &mut Vec<Row>,
+        viewport: &mut VecDeque<Row>,
         cursor: &mut Cursor,
         sixel_grid: &mut SixelGrid,
         supports_kitty_keyboard_protocol: &mut bool,
diff --git a/zellij-server/src/panes/hyperlink_tracker.rs b/zellij-server/src/panes/hyperlink_tracker.rs
index 12f6843d..56bcfcdb 100644
--- a/zellij-server/src/panes/hyperlink_tracker.rs
+++ b/zellij-server/src/panes/hyperlink_tracker.rs
@@ -47,7 +47,7 @@ impl HyperlinkTracker {
         &mut self,
         ch: char,
         cursor: &Cursor,
-        viewport: &mut Vec<Row>,
+        viewport: &mut VecDeque<Row>,
         lines_above: &mut VecDeque<Row>,
         link_handler: &mut LinkHandler,
     ) {
@@ -144,7 +144,7 @@ impl HyperlinkTracker {
 
     fn finalize_and_apply(
         &mut self,
-        viewport: &mut Vec<Row>,
+        viewport: &mut VecDeque<Row>,
         lines_above: &mut VecDeque<Row>,
         link_handler: &mut LinkHandler,
     ) {
@@ -183,7 +183,7 @@ impl HyperlinkTracker {
     fn apply_hyperlink_to_grid(
         &self,
         link: &DetectedLink,
-        viewport: &mut Vec<Row>,
+        viewport: &mut VecDeque<Row>,
         lines_above: &mut VecDeque<Row>,
         link_handler: &mut LinkHandler,
     ) {
@@ -319,7 +319,7 @@ mod tests {
         }
     }
 
-    fn create_test_viewport(rows: usize, cols: usize) -> Vec<Row> {
+    fn create_test_viewport(rows: usize, cols: usize) -> VecDeque<Row> {
         (0..rows).map(|_| create_test_row(cols)).collect()
     }
 
diff --git a/zellij-server/src/panes/search.rs b/zellij-server/src/panes/search.rs
index 5c410ad6..80a2bf5f 100644
--- a/zellij-server/src/panes/search.rs
+++ b/zellij-server/src/panes/search.rs
@@ -2,6 +2,7 @@ use super::Selection;
 use crate::panes::terminal_character::TerminalCharacter;
 use crate::panes::{Grid, Row};
 use std::borrow::Cow;
+use std::collections::VecDeque;
 use std::fmt::Debug;
 use zellij_utils::input::actions::SearchDirection;
 use zellij_utils::position::Position;
@@ -317,7 +318,7 @@ impl SearchResult {
     pub(crate) fn move_down(
         &mut self,
         amount: usize,
-        viewport: &[Row],
+        viewport: &VecDeque<Row>,
         grid_height: usize,
     ) -> bool {
         let mut found_something = false;
@@ -331,7 +332,7 @@ impl SearchResult {
 
         // Search the new line for our needle
         if !self.needle.is_empty() {
-            if let Some(row) = viewport.first() {
+            if let Some(row) = viewport.front() {
                 let mut tail = Vec::new();
                 loop {
                     let tail_idx = 1 + tail.len();
@@ -354,8 +355,8 @@ impl SearchResult {
     pub(crate) fn move_up(
         &mut self,
         amount: usize,
-        viewport: &[Row],
-        lines_below: &[Row],
+        viewport: &VecDeque<Row>,
+        lines_below: &VecDeque<Row>,
         grid_height: usize,
     ) -> bool {
         let mut found_something = false;
@@ -368,7 +369,7 @@ impl SearchResult {
 
         // Search the new line for our needle
         if !self.needle.is_empty() {
-            if let Some(row) = viewport.last() {
+            if let Some(row) = viewport.back() {
                 let tail: Vec<&Row> = lines_below.iter().take_while(|r| !r.is_canonical).collect();
                 let selections = self.search_row(viewport.len() - 1, row, &tail);
                 for selection in selections {
diff --git a/zellij-server/src/panes/unit/grid_tests.rs b/zellij-server/src/panes/unit/grid_tests.rs
index 80b413f1..310c3dc4 100644
--- a/zellij-server/src/panes/unit/grid_tests.rs
+++ b/zellij-server/src/panes/unit/grid_tests.rs
@@ -1,7 +1,7 @@
 use super::super::Grid;
 use crate::panes::grid::SixelImageStore;
 use crate::panes::link_handler::LinkHandler;
-use ::insta::assert_snapshot;
+use insta::assert_snapshot;
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::rc::Rc;