Add an environment variable for testing.

druid-shell/src/platform/x11/application.rs

@@ -91,13 +91,20 @@ impl Application {
             quitting: false,
             windows: HashMap::new(),
         }));
-        let present_opcode = match Application::query_present_opcode(&connection) {
-            Ok(p) => p,
-            Err(e) => {
-                log::info!("failed to find Present extension: {}", e);
-                None
+
+        let present_opcode = if std::env::var_os("DRUID_SHELL_DISABLE_X11_PRESENT").is_some() {
+            // Allow disabling Present with an environment variable.
+            None
+        } else {
+            match Application::query_present_opcode(&connection) {
+                Ok(p) => p,
+                Err(e) => {
+                    log::info!("failed to find Present extension: {}", e);
+                    None
+                }
             }
         };
+
         Ok(Application {
             connection,
             screen_num: screen_num as i32,

druid-shell/src/platform/x11/window.rs

@@ -579,6 +579,8 @@ impl Window {
             // We aren't using the present extension, so fall back to sleeping for scheduling
             // redraws. Sleeping is a terrible way to schedule redraws, but hopefully we don't
             // have to fall back to this very often.
+            // TODO: once we have an idle handler, we should use that. Sleeping causes lots of
+            // problems when windows are dragged to resize.
             if anim && self.refresh_rate.is_some() {
                 let sleep_amount_ms = (1000.0 / self.refresh_rate.unwrap()) as u64;
                 std::thread::sleep(std::time::Duration::from_millis(sleep_amount_ms));