test: add alternate screen integration tests

Add tests simulating vim/vi behavior to verify:
- Alternate screen shows only alternate content
- Main screen content is restored on exit
- Multiple enter/exit cycles work correctly
- Scrollback history doesn't leak into alternate view

Author: andrewgazelka

crates/tap-server/src/scrollback.rs

@@ -109,4 +109,129 @@ mod tests {
         assert!(!content.contains("\x1b[31m"));
         assert!(!content.contains("[31m"));
     }
+
+    // =========================================================================
+    // Alternate screen integration tests (simulating vim/vi behavior)
+    // =========================================================================
+
+    /// When entering alternate screen, only alternate content should be visible
+    #[test]
+    fn test_alternate_screen_shows_only_alternate_content() {
+        let mut buf = ScrollbackBuffer::new();
+
+        // Write content to main screen
+        buf.push(b"main line 1\r\nmain line 2\r\nmain line 3");
+
+        // Enter alternate screen (like vim does)
+        buf.push(b"\x1b[?1049h");
+
+        // Write content to alternate screen
+        buf.push(b"alternate content here");
+
+        // Should show ONLY alternate content, NOT main content
+        let content = buf.get_lines(None);
+        assert!(
+            content.contains("alternate content here"),
+            "should contain alternate content"
+        );
+        assert!(
+            !content.contains("main line 1"),
+            "should NOT contain main screen content when in alternate mode"
+        );
+        assert!(
+            !content.contains("main line 2"),
+            "should NOT contain main screen content when in alternate mode"
+        );
+    }
+
+    /// Simulates vim workflow: enter alternate, show file, exit, restore main
+    #[test]
+    fn test_vim_like_workflow() {
+        let mut buf = ScrollbackBuffer::new();
+
+        // User types some commands in shell
+        buf.push(b"$ ls -la\r\nfile1.txt\r\nfile2.txt\r\n$ vim file1.txt\r\n");
+
+        // Vim enters alternate screen
+        buf.push(b"\x1b[?1049h");
+
+        // Vim shows file content (simplified)
+        buf.push(b"Hello from file1.txt\r\nThis is vim editing mode");
+
+        // While in vim, should only see vim content
+        let content_in_vim = buf.get_lines(None);
+        assert!(content_in_vim.contains("Hello from file1.txt"));
+        assert!(
+            !content_in_vim.contains("$ ls -la"),
+            "shell history should not be visible while in vim"
+        );
+
+        // User exits vim (:q)
+        buf.push(b"\x1b[?1049l");
+
+        // Back to main screen, should see original shell content
+        let content_after_vim = buf.get_lines(None);
+        assert!(
+            content_after_vim.contains("$ ls -la"),
+            "shell history should be restored after exiting vim"
+        );
+        assert!(
+            content_after_vim.contains("file1.txt"),
+            "ls output should be visible after exiting vim"
+        );
+    }
+
+    /// Test multiple alternate screen enter/exit cycles
+    #[test]
+    fn test_multiple_alternate_screen_cycles() {
+        let mut buf = ScrollbackBuffer::new();
+
+        // Initial main screen content
+        buf.push(b"session start\r\n");
+
+        for i in 1..=3 {
+            // Enter alternate screen
+            buf.push(b"\x1b[?1049h");
+            buf.push(format!("editor session {i}").as_bytes());
+
+            // Should only see current editor session
+            let in_alt = buf.get_lines(None);
+            assert!(in_alt.contains(&format!("editor session {i}")));
+            assert!(!in_alt.contains("session start"));
+
+            // Exit alternate screen
+            buf.push(b"\x1b[?1049l");
+
+            // Should be back to main
+            let in_main = buf.get_lines(None);
+            assert!(in_main.contains("session start"));
+        }
+    }
+
+    /// Test that alternate screen content is isolated
+    #[test]
+    fn test_alternate_screen_isolation() {
+        let mut buf = ScrollbackBuffer::new();
+
+        // Main screen has scrollback history
+        for i in 1..=50 {
+            buf.push(format!("history line {i}\r\n").as_bytes());
+        }
+
+        // Enter alternate screen (TUI app)
+        buf.push(b"\x1b[?1049h");
+        buf.push(b"TUI application interface");
+
+        // Alt-e equivalent: get_lines should return ONLY the TUI content
+        let content = buf.get_lines(None);
+        assert!(content.contains("TUI application interface"));
+        assert!(
+            !content.contains("history line 1"),
+            "scrollback history should not leak into alternate screen view"
+        );
+        assert!(
+            !content.contains("history line 50"),
+            "scrollback history should not leak into alternate screen view"
+        );
+    }
 }