fix: do not re-render unless needed (#7186)

Previously, dune would re-render on every frame even when it wasn't
necessary.

Now, dune will make sure we have at least one modification before
re-rendering

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

CHANGES.md

@@ -1,6 +1,9 @@
 Unreleased
 ----------
 
+- Do not re-render UI on every frame if the UI doesn't change (#7186, fix
+  #7184, @rgrinberg)
+
 - Fix preludes not being recorded as dependencies in the `(mdx)` stanza (#7109,
   fixes #7077, @emillon).
 

src/dune_threaded_console/dune_threaded_console.ml

@@ -16,10 +16,12 @@ let make (module Base : S) : (module Dune_console.Backend) =
       ; status_line = None
       ; finished = false
       ; finish_requested = false
+      ; dirty = true
       }
 
     let finish () =
       Mutex.lock mutex;
+      state.dirty <- true;
       state.finish_requested <- true;
       while not state.finished do
         Condition.wait finish_cv mutex
@@ -28,25 +30,29 @@ let make (module Base : S) : (module Dune_console.Backend) =
 
     let print_user_message m =
       Mutex.lock mutex;
+      state.dirty <- true;
       Queue.push state.messages m;
       Mutex.unlock mutex
 
     let set_status_line sl =
       Mutex.lock mutex;
+      state.dirty <- true;
       state.status_line <- sl;
       Mutex.unlock mutex
 
     let print_if_no_status_line _msg = ()
 
     let reset () =
       Mutex.lock mutex;
+      state.dirty <- true;
       Queue.clear state.messages;
       state.status_line <- None;
       Base.reset ();
       Mutex.unlock mutex
 
     let reset_flush_history () =
       Mutex.lock mutex;
+      state.dirty <- true;
       Queue.clear state.messages;
       state.status_line <- None;
       Base.reset_flush_history ();
@@ -84,9 +90,13 @@ let make (module Base : S) : (module Dune_console.Backend) =
            events and sleep for the remaining time. *)
         while true do
           Mutex.lock mutex;
-          Base.render state;
-          let finish_requested = state.finish_requested in
-          if finish_requested then raise_notrace Exit;
+          (match state.dirty with
+          | false -> ()
+          | true ->
+            Base.render state;
+            let finish_requested = state.finish_requested in
+            if finish_requested then raise_notrace Exit;
+            state.dirty <- false);
           Mutex.unlock mutex;
           let now = Unix.gettimeofday () in
           let elapsed = now -. !last in

src/dune_threaded_console/dune_threaded_console_intf.ml

@@ -7,6 +7,7 @@ type state =
   ; mutable finish_requested : bool
   ; mutable finished : bool
   ; mutable status_line : User_message.Style.t Pp.t option
+  ; mutable dirty : bool
   }
 
 (** [Threaded] is the interface for user interfaces that are rendered in a