perf: send scrolling pane output as row shifts and skip blank cells (#4711)

* perf: send scrolling pane output as row shifts and skip blank cells

* fix: keep expanded scroll patches disjoint for the fast presentation path

2 files changed, 18 insertions, 4 deletions

diff --git a/src/pane/terminal.rs b/src/pane/terminal.rs
index 8ac4bd1c..24379d22 100644
--- a/src/pane/terminal.rs
+++ b/src/pane/terminal.rs
@@ -2677,6 +2677,7 @@ fn ghostty_collect_dirty_patch(
     let mut grapheme_bytes = Vec::new();
     let mut symbol_scratch = String::new();
     let mut patch_rows = Vec::new();
+    let blank = blank_cell_data(default_fg, default_bg);
     while let Some(y) = rows.next_dirty() {
         if y >= area_height {
             break;
@@ -2692,6 +2693,15 @@ fn ghostty_collect_dirty_patch(
         let mut patch_cells = Vec::with_capacity(usize::from(area_width));
         let mut x = 0u16;
         while x < area_width && cells.next() {
+            match cells.is_default_blank() {
+                Ok(true) => {
+                    patch_cells.push(blank.clone());
+                    x += 1;
+                    continue;
+                }
+                Ok(false) => {}
+                Err(_) => fallback!("raw_cell_error"),
+            }
             let Ok(basic) = cells.basic_data() else {
                 fallback!("basic_data_error");
             };
@@ -2720,10 +2730,7 @@ fn ghostty_collect_dirty_patch(
             patch_cells.push(cell_data_from_style(symbol, style));
             x += 1;
         }
-        while x < area_width {
-            patch_cells.push(blank_cell_data(default_fg, default_bg));
-            x += 1;
-        }
+        patch_cells.resize(usize::from(area_width), blank.clone());
         patch_rows.push((y, patch_cells));
     }
 
diff --git a/src/pane/terminal/migration_tests.rs b/src/pane/terminal/migration_tests.rs
index 09e5df01..aaf91f27 100644
--- a/src/pane/terminal/migration_tests.rs
+++ b/src/pane/terminal/migration_tests.rs
@@ -311,6 +311,13 @@ fn incremental_rows_reconstruct_full_render() {
         b"\x1b[3;4H",
         b"\x1b[?1049hALT",
         b"\x1b[?1049l",
+        // Blank cells must match the full renderer under every color source.
+        b"\x1b[3;1H\x1b[44m\x1b[2K\x1b[0m",
+        b"\x1b[4;2H\x1b[7m \x1b[0m \x1b[8m \x1b[0m",
+        b"\x1b[?5h",
+        b"\x1b[?5l",
+        b"\x1b[5;1H\x1b]133;A\x07$ \x1b]133;B\x07  \x1b]133;C\x07",
+        b"\x1b[5;1H\r\n\r\nscrolled\r\n",
     ] {
         incremental.write(bytes);
         full.write(bytes);