Make backend persistence opt-in

cursive-core/src/backend.rs

@@ -81,6 +81,14 @@ pub trait Backend {
     /// Any call to `print_at` from now on should use the given effect.
     fn set_effect(&self, effect: theme::Effect);
 
+    /// Returns `true` if the backend has persistent output.
+    ///
+    /// If true, this means that output stays there between calls to
+    /// `refresh()`, so only delta needs to be sent.
+    fn is_persistent(&self) -> bool {
+        false
+    }
+
     /// Disables the given effect.
     fn unset_effect(&self, effect: theme::Effect);
 

cursive/src/backends/crossterm.rs

@@ -327,6 +327,10 @@ impl Drop for Backend {
 }
 
 impl backend::Backend for Backend {
+    fn is_persistent(&self) -> bool {
+        true
+    }
+
     fn poll_event(&mut self) -> Option<Event> {
         match poll(Duration::from_millis(1)) {
             Ok(true) => match read() {

cursive/src/backends/curses/n.rs

@@ -322,6 +322,10 @@ impl backend::Backend for Backend {
         "ncurses"
     }
 
+    fn is_persistent(&self) -> bool {
+        true
+    }
+
     fn set_title(&mut self, title: String) {
         write_to_tty(format!("\x1B]0;{title}\x07").as_bytes()).unwrap();
     }

cursive/src/backends/curses/pan.rs

@@ -377,6 +377,10 @@ impl backend::Backend for Backend {
         "pancurses"
     }
 
+    fn is_persistent(&self) -> bool {
+        true
+    }
+
     fn set_title(&mut self, title: String) {
         print!("\x1B]0;{title}\x07");
         stdout().flush().expect("could not flush stdout");

cursive/src/backends/termion.rs

@@ -267,6 +267,10 @@ impl backend::Backend for Backend {
         "termion"
     }
 
+    fn is_persistent(&self) -> bool {
+        true
+    }
+
     fn set_title(&mut self, title: String) {
         write!(self.terminal.get_mut(), "\x1B]0;{title}\x07").unwrap();
     }