Add support for scrolling via accesskit / kittest (#7286)

I need to scroll in a snapshot test in my app, and kittest had no
utilities for this. Event::MouseWheel is error prone. This adds support
for some accesskit scroll actions, and uses this in kittest to add
helpers to scroll to a node / scroll the scroll area surrounding a node.

The accesskit code says down/up/left/right `Scrolls by approximately one
screen in a specific direction.`. Unfortunately it's difficult to get
the size of a "screen" (I guess that would be the size of the containing
scroll area)where I implemented the scrolling, so for now I've hardcoded
it to 100px. I think scrolling a fixed amount is still better than not
scrolling at all.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/context.rs

@@ -1149,7 +1149,7 @@ impl Context {
                              ID clashes happens when things like Windows or CollapsingHeaders share names,\n\
                              or when things like Plot and Grid:s aren't given unique id_salt:s.\n\n\
                              Sometimes the solution is to use ui.push_id.",
-                         if below { "above" } else { "below" })
+                                if below { "above" } else { "below" }),
                     );
                 }
             }
@@ -1216,6 +1216,51 @@ impl Context {
             self.accesskit_node_builder(w.id, |builder| res.fill_accesskit_node_common(builder));
         }
 
+        #[cfg(feature = "accesskit")]
+        self.write(|ctx| {
+            use crate::{Align, pass_state::ScrollTarget, style::ScrollAnimation};
+            let viewport = ctx.viewport_for(ctx.viewport_id());
+
+            viewport
+                .input
+                .consume_accesskit_action_requests(res.id, |request| {
+                    // TODO(lucasmerlin): Correctly handle the scroll unit:
+                    // https://github.com/AccessKit/accesskit/blob/e639c0e0d8ccbfd9dff302d972fa06f9766d608e/common/src/lib.rs#L2621
+                    const DISTANCE: f32 = 100.0;
+
+                    match &request.action {
+                        accesskit::Action::ScrollIntoView => {
+                            viewport.this_pass.scroll_target = [
+                                Some(ScrollTarget::new(
+                                    res.rect.x_range(),
+                                    Some(Align::Center),
+                                    ScrollAnimation::none(),
+                                )),
+                                Some(ScrollTarget::new(
+                                    res.rect.y_range(),
+                                    Some(Align::Center),
+                                    ScrollAnimation::none(),
+                                )),
+                            ];
+                        }
+                        accesskit::Action::ScrollDown => {
+                            viewport.this_pass.scroll_delta.0 += DISTANCE * Vec2::UP;
+                        }
+                        accesskit::Action::ScrollUp => {
+                            viewport.this_pass.scroll_delta.0 += DISTANCE * Vec2::DOWN;
+                        }
+                        accesskit::Action::ScrollLeft => {
+                            viewport.this_pass.scroll_delta.0 += DISTANCE * Vec2::LEFT;
+                        }
+                        accesskit::Action::ScrollRight => {
+                            viewport.this_pass.scroll_delta.0 += DISTANCE * Vec2::RIGHT;
+                        }
+                        _ => return false,
+                    };
+                    true
+                });
+        });
+
         res
     }
 

crates/egui/src/input_state/mod.rs

@@ -824,6 +824,23 @@ impl InputState {
         })
     }
 
+    #[cfg(feature = "accesskit")]
+    pub fn consume_accesskit_action_requests(
+        &mut self,
+        id: crate::Id,
+        mut consume: impl FnMut(&accesskit::ActionRequest) -> bool,
+    ) {
+        let accesskit_id = id.accesskit_id();
+        self.events.retain(|event| {
+            if let Event::AccessKitActionRequest(request) = event {
+                if request.target == accesskit_id {
+                    return !consume(request);
+                }
+            }
+            true
+        });
+    }
+
     #[cfg(feature = "accesskit")]
     pub fn has_accesskit_action_request(&self, id: crate::Id, action: accesskit::Action) -> bool {
         self.accesskit_action_requests(id, action).next().is_some()