terminal: Reduce flicker on resize (#47195)

Skip PTY resizes for pixel-only changes and coalesce pending resize
events.
Snap standalone terminal layout to whole device pixels to avoid subpixel
jitter.

before:


https://github.com/user-attachments/assets/0ad0db83-0099-44c8-b8d1-3dc8146b25ef


after:


https://github.com/user-attachments/assets/86278014-1c87-4263-a9e5-b58bcc1fa2ea




Release Notes:

- Fixed: Reduce terminal flicker on resize

---------

Signed-off-by: pigletfly <wangbing.adam@gmail.com>
Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/terminal/src/terminal.rs

@@ -1440,8 +1440,27 @@ impl Terminal {
 
     ///Resize the terminal and the PTY.
     pub fn set_size(&mut self, new_bounds: TerminalBounds) {
-        if self.last_content.terminal_bounds != new_bounds {
-            self.events.push_back(InternalEvent::Resize(new_bounds))
+        let mut new_bounds = new_bounds;
+        new_bounds.bounds.size.height = cmp::max(new_bounds.line_height, new_bounds.height());
+        new_bounds.bounds.size.width = cmp::max(new_bounds.cell_width, new_bounds.width());
+
+        let old_bounds = self.last_content.terminal_bounds;
+        self.last_content.terminal_bounds = new_bounds;
+
+        // Avoid spamming PTY resizes on pixel-level size changes (e.g. while dragging edges),
+        // since those can generate excessive SIGWINCH/reflows and cause visible flicker.
+        let requires_resize = old_bounds.num_lines() != new_bounds.num_lines()
+            || old_bounds.num_columns() != new_bounds.num_columns()
+            || old_bounds.cell_width != new_bounds.cell_width
+            || old_bounds.line_height != new_bounds.line_height;
+
+        if !requires_resize {
+            return;
+        }
+
+        match self.events.back_mut() {
+            Some(InternalEvent::Resize(pending_bounds)) => *pending_bounds = new_bounds,
+            _ => self.events.push_back(InternalEvent::Resize(new_bounds)),
         }
     }
 
@@ -3036,6 +3055,51 @@ mod tests {
         );
     }
 
+    #[gpui::test]
+    async fn test_set_size_coalesces_pixel_only_changes(cx: &mut TestAppContext) {
+        let builder = cx.update(|cx| {
+            TerminalBuilder::new_display_only(
+                CursorShape::Block,
+                AlternateScroll::On,
+                None,
+                0,
+                cx.background_executor(),
+                PathStyle::local(),
+            )
+            .unwrap()
+        });
+        let mut terminal = builder.terminal;
+
+        let base_bounds = TerminalBounds {
+            cell_width: Pixels::from(10.),
+            line_height: Pixels::from(10.),
+            bounds: bounds(
+                Point::default(),
+                size(Pixels::from(100.), Pixels::from(100.)),
+            ),
+        };
+
+        terminal.set_size(base_bounds);
+        terminal.events.clear();
+        assert_eq!(terminal.last_content.terminal_bounds, base_bounds);
+
+        // Pixel-only change: height grows by 1px but still the same number of rows/cols.
+        let mut pixel_changed = base_bounds;
+        pixel_changed.bounds.size.height = Pixels::from(101.);
+        terminal.set_size(pixel_changed);
+        assert!(terminal.events.is_empty());
+        assert_eq!(terminal.last_content.terminal_bounds, pixel_changed);
+
+        // Grid change: height increases enough to add a row.
+        let mut grid_changed = base_bounds;
+        grid_changed.bounds.size.height = Pixels::from(110.);
+        terminal.set_size(grid_changed);
+        assert!(matches!(
+            terminal.events.back(),
+            Some(InternalEvent::Resize(_))
+        ));
+    }
+
     fn get_cells(size: TerminalBounds, rng: &mut StdRng) -> Vec<Vec<char>> {
         let mut cells = Vec::new();
 

crates/terminal_view/src/terminal_element.rs

@@ -50,7 +50,6 @@ pub struct LayoutState {
     mode: TermMode,
     display_offset: usize,
     hyperlink_tooltip: Option<AnyElement>,
-    gutter: Pixels,
     block_below_cursor_element: Option<AnyElement>,
     base_text_style: TextStyle,
     content_mode: ContentMode,
@@ -970,6 +969,7 @@ impl Element for TerminalElement {
 
                     let mut size = bounds.size;
                     size.width -= gutter;
+                    let available_height = size.height;
 
                     // https://github.com/zed-industries/zed/issues/2750
                     // if the terminal is one column wide, rendering 🦀
@@ -981,6 +981,43 @@ impl Element for TerminalElement {
                     let mut origin = bounds.origin;
                     origin.x += gutter;
 
+                    if matches!(self.terminal_view.read(cx).mode, TerminalMode::Standalone) {
+                        let scale_factor = window.scale_factor();
+                        let line_height_pixels = px(line_height);
+                        let line_height_device_px = (f32::from(line_height_pixels) * scale_factor)
+                            .round()
+                            .max(1.0) as i32;
+                        let available_height_device_px =
+                            (f32::from(available_height) * scale_factor)
+                                .floor()
+                                .max(0.0) as i32;
+
+                        let rows =
+                            ((available_height_device_px / line_height_device_px) as usize).max(1);
+                        let snapped_height_device_px = (rows as i32) * line_height_device_px;
+                        let padding_device_px =
+                            (available_height_device_px - snapped_height_device_px).max(0);
+
+                        let snapped_height =
+                            px(snapped_height_device_px as f32 / scale_factor.max(1.0));
+                        let padding = px(padding_device_px as f32 / scale_factor.max(1.0));
+
+                        size.height = snapped_height;
+                        if self.terminal.read(cx).scrolled_to_bottom() {
+                            origin.y += padding;
+                        }
+                    }
+
+                    // Snap to device pixels to avoid subpixel jitter while resizing.
+                    // Terminal rendering is grid-based; allowing fractional origins can cause the
+                    // glyph rasterization to shift between frames, which looks like flicker.
+                    let scale_factor = window.scale_factor();
+                    let snap_px = |value: Pixels| {
+                        Pixels::from((f32::from(value) * scale_factor).floor() / scale_factor)
+                    };
+                    origin.x = snap_px(origin.x);
+                    origin.y = snap_px(origin.y);
+
                     (
                         TerminalBounds::new(px(line_height), cell_width, Bounds { origin, size }),
                         line_height,
@@ -1018,7 +1055,7 @@ impl Element for TerminalElement {
 
                 let scroll_top = self.terminal_view.read(cx).scroll_top;
                 let hyperlink_tooltip = hover_tooltip.map(|hover_tooltip| {
-                    let offset = bounds.origin + point(gutter, px(0.)) - point(px(0.), scroll_top);
+                    let offset = dimensions.bounds.origin - point(px(0.), scroll_top);
                     let mut element = div()
                         .size_full()
                         .id("terminal-element")
@@ -1062,8 +1099,9 @@ impl Element for TerminalElement {
                 //
                 // This optimization is analogous to the editor optimization in PR #45077
                 // which fixed performance issues with large AutoHeight editors inside Lists.
+                let content_bounds = dimensions.bounds;
                 let visible_bounds = window.content_mask().bounds;
-                let intersection = visible_bounds.intersect(&bounds);
+                let intersection = visible_bounds.intersect(&content_bounds);
 
                 // If the terminal is entirely outside the viewport, skip all cell processing.
                 // This handles the case where the terminal has been scrolled past (above or
@@ -1073,7 +1111,7 @@ impl Element for TerminalElement {
                     || intersection.size.width <= px(0.)
                 {
                     (Vec::new(), Vec::new())
-                } else if intersection == bounds {
+                } else if intersection == content_bounds {
                     // Fast path: terminal fully visible, no clipping needed.
                     // Avoid grouping/allocation overhead by streaming cells directly.
                     TerminalElement::layout_grid(
@@ -1092,9 +1130,9 @@ impl Element for TerminalElement {
                     // by screen position (enumerated line group index), not by the cell's
                     // internal line number (which can be negative in Scrollable mode for
                     // scrollback history).
-                    let rows_above_viewport =
-                        f32::from((intersection.top() - bounds.top()).max(px(0.)) / line_height_px)
-                            as usize;
+                    let rows_above_viewport = f32::from(
+                        (intersection.top() - content_bounds.top()).max(px(0.)) / line_height_px,
+                    ) as usize;
                     let visible_row_count =
                         f32::from((intersection.size.height / line_height_px).ceil()) as usize + 1;
 
@@ -1199,7 +1237,7 @@ impl Element for TerminalElement {
                                 block.height as f32 * dimensions.line_height(),
                             ),
                         );
-                        let origin = bounds.origin
+                        let origin = Point::new(bounds.origin.x, dimensions.bounds.origin.y)
                             + point(px(0.), target_line as f32 * dimensions.line_height())
                             - point(px(0.), scroll_top);
                         window.with_rem_size(rem_size, |window| {
@@ -1225,7 +1263,6 @@ impl Element for TerminalElement {
                     mode,
                     display_offset,
                     hyperlink_tooltip,
-                    gutter,
                     block_below_cursor_element,
                     base_text_style: text_style,
                     content_mode,
@@ -1249,8 +1286,12 @@ impl Element for TerminalElement {
             let scroll_top = self.terminal_view.read(cx).scroll_top;
 
             window.paint_quad(fill(bounds, layout.background_color));
-            let origin =
-                bounds.origin + Point::new(layout.gutter, px(0.)) - Point::new(px(0.), scroll_top);
+            let origin = layout.dimensions.bounds.origin - Point::new(px(0.), scroll_top);
+            let scale_factor = window.scale_factor();
+            let snap_px = |value: Pixels| {
+                Pixels::from((f32::from(value) * scale_factor).floor() / scale_factor)
+            };
+            let origin = point(snap_px(origin.x), snap_px(origin.y));
 
             let marked_text_cloned: Option<String> = {
                 let ime_state = &self.terminal_view.read(cx).ime_state;