Add cursor grab for web target (#2025)

* Add cursor grab

* Update feature matrix, changelog and platform information

* Add proper error propagation

* Remove "expect" from fallible code

    code would crash if handling pointer capture outside of winit on
    every mouse click

    we swallow the error since we could not think of a case where this
    would fail in a way that would want to handle that it fails

* Remove unnecessary implementation comment

Co-authored-by: Will Crichton <wcrichto@cs.stanford.edu>

src/platform_impl/web/web_sys/canvas.rs

@@ -89,6 +89,20 @@ impl Canvas {
         })
     }
 
+    pub fn set_cursor_grab(&self, grab: bool) -> Result<(), RootOE> {
+        if grab {
+            self.raw().request_pointer_lock();
+        } else {
+            let window = web_sys::window()
+                .ok_or(os_error!(OsError("Failed to obtain window".to_owned())))?;
+            let document = window
+                .document()
+                .ok_or(os_error!(OsError("Failed to obtain document".to_owned())))?;
+            document.exit_pointer_lock();
+        }
+        Ok(())
+    }
+
     pub fn set_attribute(&self, attribute: &str, value: &str) {
         self.common
             .raw

src/platform_impl/web/web_sys/canvas/pointer_handler.rs

@@ -79,9 +79,11 @@ impl PointerHandler {
                     event::mouse_button(&event),
                     event::mouse_modifiers(&event),
                 );
-                canvas
-                    .set_pointer_capture(event.pointer_id())
-                    .expect("Failed to set pointer capture");
+
+                // Error is swallowed here since the error would occur every time the mouse is
+                // clicked when the cursor is grabbed, and there is probably not a situation where
+                // this could fail, that we care if it fails.
+                let _e = canvas.set_pointer_capture(event.pointer_id());
             },
         ));
     }