Add `OutputCommand::SetPointerPosition` to set mouse position (#5776)

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This is not supported on the web and not yet on Wayland.

~~I also had to update `ring` and add an exception for `paste` being
unmaintained.~~ Has since been updated on master.

* [x] I have followed the instructions in the PR template

crates/eframe/src/web/app_runner.rs

@@ -336,6 +336,9 @@ impl AppRunner {
                 egui::OutputCommand::OpenUrl(open_url) => {
                     super::open_url(&open_url.url, open_url.new_tab);
                 }
+                egui::OutputCommand::SetPointerPosition(_) => {
+                    // Not supported on the web.
+                }
             }
         }
 

crates/egui-winit/src/lib.rs

@@ -850,6 +850,9 @@ impl State {
                 egui::OutputCommand::OpenUrl(open_url) => {
                     open_url_in_browser(&open_url.url);
                 }
+                egui::OutputCommand::SetPointerPosition(egui::Pos2 { x, y }) => {
+                    let _ = window.set_cursor_position(winit::dpi::LogicalPosition { x, y });
+                }
             }
         }
 

crates/egui/src/context.rs

@@ -1482,6 +1482,11 @@ impl Context {
         self.send_cmd(crate::OutputCommand::CopyImage(image));
     }
 
+    /// Set the mouse cursor position (if the platform supports it).
+    pub fn set_pointer_position(&self, position: Pos2) {
+        self.send_cmd(crate::OutputCommand::SetPointerPosition(position));
+    }
+
     /// Format the given shortcut in a human-readable way (e.g. `Ctrl+Shift+X`).
     ///
     /// Can be used to get the text for [`crate::Button::shortcut_text`].

crates/egui/src/data/output.rs

@@ -95,6 +95,9 @@ pub enum OutputCommand {
 
     /// Open this url in a browser.
     OpenUrl(OpenUrl),
+
+    /// Set the mouse cursor position (if the platform supports it).
+    SetPointerPosition(emath::Pos2),
 }
 
 /// The non-rendering part of what egui emits each frame.

crates/egui_demo_lib/src/demo/tests/cursor_test.rs

@@ -16,6 +16,14 @@ impl crate::Demo for CursorTest {
 
 impl crate::View for CursorTest {
     fn ui(&mut self, ui: &mut egui::Ui) {
+        if ui
+            .button("Center pointer in window")
+            .on_hover_text("The platform may not support this.")
+            .clicked()
+        {
+            let position = ui.ctx().available_rect().center();
+            ui.ctx().set_pointer_position(position);
+        }
         ui.vertical_centered_justified(|ui| {
             ui.heading("Hover to switch cursor icon:");
             for &cursor_icon in &egui::CursorIcon::ALL {