term: hook flush up to ThreadedWriter

I'd forgotten that this was in here.
The recentish fixes to flush for various terminal queries
likely need this to be 100% good.  Surprised this hasn't
come up as a bug already :-/

term/src/terminalstate/mod.rs

@@ -436,17 +436,31 @@ fn default_color_map() -> HashMap<u16, RgbColor> {
 /// and we're in control of the write side, which represents
 /// input from the interactive user, or pastes.
 struct ThreadedWriter {
-    sender: Sender<Vec<u8>>,
+    sender: Sender<WriterMessage>,
+}
+
+enum WriterMessage {
+    Data(Vec<u8>),
+    Flush,
 }
 
 impl ThreadedWriter {
     fn new(mut writer: Box<dyn std::io::Write + Send>) -> Self {
-        let (sender, receiver) = channel::<Vec<u8>>();
+        let (sender, receiver) = channel::<WriterMessage>();
 
         std::thread::spawn(move || {
-            while let Ok(buf) = receiver.recv() {
-                if writer.write(&buf).is_err() {
-                    break;
+            while let Ok(msg) = receiver.recv() {
+                match msg {
+                    WriterMessage::Data(buf) => {
+                        if writer.write(&buf).is_err() {
+                            break;
+                        }
+                    }
+                    WriterMessage::Flush => {
+                        if writer.flush().is_err() {
+                            break;
+                        }
+                    }
                 }
             }
         });
@@ -458,11 +472,15 @@ impl ThreadedWriter {
 impl std::io::Write for ThreadedWriter {
     fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
         self.sender
-            .send(buf.to_vec())
+            .send(WriterMessage::Data(buf.to_vec()))
             .map_err(|err| std::io::Error::new(std::io::ErrorKind::BrokenPipe, err))?;
         Ok(buf.len())
     }
+
     fn flush(&mut self) -> std::io::Result<()> {
+        self.sender
+            .send(WriterMessage::Flush)
+            .map_err(|err| std::io::Error::new(std::io::ErrorKind::BrokenPipe, err))?;
         Ok(())
     }
 }