Fix tmux statusline color leaking into alt screen

app/src/terminal/grid_renderer.rs

@@ -1134,6 +1134,11 @@ fn render_grid_with_ligatures<'a>(
             None => {
                 // If there are no non-empty cells in the entire row, we can skip it entirely
                 if marked_text.peek().is_none() {
+                    if let Some(sampler) = bg_color_sampler.as_deref_mut() {
+                        for _ in 0..grid.columns() {
+                            sampler.sample(ColorU::transparent_black());
+                        }
+                    }
                     continue;
                 }
                 grid.columns() - 1

app/src/terminal/model/terminal_model.rs

@@ -2049,8 +2049,10 @@ impl TerminalModel {
                 .active_block_mut()
                 .set_saved_cursor(block_list_cursor);
 
-            // Reset alt screen contents.
-            let bg = self.alt_screen.grid_storage().cursor().template.bg;
+            // Reset alt screen contents. DECSET 1049 starts with a fresh
+            // alternate buffer; a transient SGR background from UI chrome like
+            // tmux's status line should not seed the entire pane.
+            let bg = ansi::Color::Named(ansi::NamedColor::Background);
             self.alt_screen
                 .grid_storage_mut()
                 .region_mut(..)

app/src/terminal/model/terminal_model_test.rs

@@ -650,6 +650,31 @@ fn test_alt_screen_toggle() {
     assert!(!terminal.alt_screen_active);
 }
 
+#[test]
+fn test_enter_alt_screen_clears_with_default_background() {
+    let mut terminal = TerminalModel::mock(None, None);
+
+    terminal.terminal_attribute(ansi::Attr::Background(ansi::Color::Named(
+        ansi::NamedColor::Green,
+    )));
+    terminal.set_mode(ansi::Mode::SwapScreen {
+        save_cursor_and_clear_screen: true,
+    });
+
+    let grid = terminal.alt_screen().grid_handler();
+    for row in 0..grid.visible_rows() {
+        let row = grid.row(row).expect("visible row should exist");
+        for cell in &row[..] {
+            assert_eq!(cell.c, '\0');
+            assert_eq!(
+                cell.bg,
+                ansi::Color::Named(ansi::NamedColor::Background),
+                "alternate screen should not inherit transient primary-screen SGR background"
+            );
+        }
+    }
+}
+
 #[test]
 fn test_reset_state() {
     let mut terminal: TerminalModel = TerminalModel::mock(None, None);