Add debug option to skip cursor-only updates while VRR is active (#1616)

* Add debug option to skip cursor-only updates while VRR is active

* Update niri-config/src/lib.rs

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

* Update src/backend/tty.rs

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

* Update wiki/Configuration:-Debug-Options.md

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

* Update Configuration:-Debug-Options.md

* Update tty.rs

* Update lib.rs

* Update Configuration:-Debug-Options.md

---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

niri-config/src/lib.rs

@@ -2329,6 +2329,8 @@ pub struct DebugConfig {
     pub strict_new_window_focus_policy: bool,
     #[knuffel(child)]
     pub honor_xdg_activation_with_invalid_serial: bool,
+    #[knuffel(child)]
+    pub skip_cursor_only_updates_during_vrr: bool,
 }
 
 #[derive(knuffel::DecodeScalar, Debug, Clone, Copy, PartialEq, Eq)]
@@ -5286,6 +5288,7 @@ mod tests {
                 disable_monitor_names: false,
                 strict_new_window_focus_policy: false,
                 honor_xdg_activation_with_invalid_serial: false,
+                skip_cursor_only_updates_during_vrr: false,
             },
             workspaces: [
                 Workspace {

src/backend/tty.rs

@@ -1422,6 +1422,12 @@ impl Tty {
             if debug.disable_cursor_plane {
                 flags.remove(FrameFlags::ALLOW_CURSOR_PLANE_SCANOUT);
             }
+            if debug.skip_cursor_only_updates_during_vrr {
+                let output_state = niri.output_state.get(output).unwrap();
+                if output_state.frame_clock.vrr() {
+                    flags.insert(FrameFlags::SKIP_CURSOR_ONLY_UPDATES);
+                }
+            }
 
             flags
         };

wiki/Configuration:-Debug-Options.md

@@ -29,6 +29,7 @@ debug {
     disable-monitor-names
     strict-new-window-focus-policy
     honor-xdg-activation-with-invalid-serial 
+    skip-cursor-only-updates-during-vrr
 }
 
 binds {
@@ -275,6 +276,22 @@ debug {
 }
 ```
 
+### `skip-cursor-only-updates-during-vrr`
+
+<sup>Since: next release</sup>
+
+Skips redrawing the screen from cursor input while variable refresh rate is active.
+
+Useful for games where the cursor isn't drawn internally to prevent erratic VRR shifts in response to cursor movement.
+
+Note that the current implementation has some issues, for example when there's nothing redrawing the screen (like a game), the rendering will appear to completely freeze (since cursor movements won't cause redraws).
+
+```kdl
+debug {
+    skip-cursor-only-updates-during-vrr
+}
+```
+
 ### Key Bindings
 
 These are not debug options, but rather key bindings.