linux/x11: Reduce input latency and ensure rerender priority (#13355)

This change ensures that we always render a window according to its
refresh rate, even if there are a lot of X11 events.

We're working around some limitations of `calloop`. In the future, we
think we should revisit how the event loop is implemented on X11, so
that we can ensure proper prioritization of input events vs. rendering.

Release Notes:

- N/A

Co-authored-by: Antonio <me@as-cii.com>

crates/gpui/src/platform/linux/x11/window.rs

@@ -14,6 +14,7 @@ use util::{maybe, ResultExt};
 use x11rb::{
     connection::Connection,
     protocol::{
+        randr::{self, ConnectionExt as _},
         xinput::{self, ConnectionExt as _},
         xproto::{
             self, ClientMessageEvent, ConnectionExt as _, EventMask, TranslateCoordinatesReply,
@@ -31,6 +32,7 @@ use std::{
     ptr::NonNull,
     rc::Rc,
     sync::{self, Arc},
+    time::{Duration, Instant},
 };
 
 use super::{X11Display, XINPUT_MASTER_DEVICE};
@@ -159,6 +161,8 @@ pub struct Callbacks {
 
 pub struct X11WindowState {
     pub destroyed: bool,
+    pub last_render_at: Option<Instant>,
+    pub refresh_rate: Duration,
     client: X11ClientStatePtr,
     executor: ForegroundExecutor,
     atoms: XcbAtoms,
@@ -389,6 +393,31 @@ impl X11WindowState {
         };
         xcb_connection.map_window(x_window).unwrap();
 
+        let screen_resources = xcb_connection
+            .randr_get_screen_resources(x_window)
+            .unwrap()
+            .reply()
+            .expect("Could not find available screens");
+
+        let mode = screen_resources
+            .crtcs
+            .iter()
+            .find_map(|crtc| {
+                let crtc_info = xcb_connection
+                    .randr_get_crtc_info(*crtc, x11rb::CURRENT_TIME)
+                    .ok()?
+                    .reply()
+                    .ok()?;
+
+                screen_resources
+                    .modes
+                    .iter()
+                    .find(|m| m.id == crtc_info.mode)
+            })
+            .expect("Unable to find screen refresh rate");
+
+        let refresh_rate = mode_refresh_rate(mode);
+
         Ok(Self {
             client,
             executor,
@@ -405,6 +434,8 @@ impl X11WindowState {
             appearance,
             handle,
             destroyed: false,
+            last_render_at: None,
+            refresh_rate,
         })
     }
 
@@ -574,6 +605,11 @@ impl X11WindowStatePtr {
         let mut cb = self.callbacks.borrow_mut();
         if let Some(ref mut fun) = cb.request_frame {
             fun();
+
+            self.state
+                .borrow_mut()
+                .last_render_at
+                .replace(Instant::now());
         }
     }
 
@@ -1020,3 +1056,12 @@ impl PlatformWindow for X11Window {
         false
     }
 }
+
+// Adatpted from:
+// https://docs.rs/winit/0.29.11/src/winit/platform_impl/linux/x11/monitor.rs.html#103-111
+pub fn mode_refresh_rate(mode: &randr::ModeInfo) -> Duration {
+    let millihertz = mode.dot_clock as u64 * 1_000 / (mode.htotal as u64 * mode.vtotal as u64);
+    let micros = 1_000_000_000 / millihertz;
+    log::info!("Refreshing at {} micros", micros);
+    Duration::from_micros(micros)
+}