Added Context::is_context_menu_open() (#3267)

I encountered a case where I needed to know if an egui context menu was
open, even if the mouse cursor was not in an egui area (full details in
discord).  I found that every resource I needed to detect that the menu
was open was either private, or pub(crate).  While it is possible to
wrap the `Response::context_menu()` in code to do state-tracking, it
becomes necessary to duplicate that code in every place you use a
context menu.

In this commit, I add `Context::is_context_menu_open()`.  Named similarly
to `Context::is_pointer_over_area()`, this method will return true if
any context menu is open.  This is possible because the context menu
uses a temp var with a fixed Id to store its state.  This method just
fetches the state, and interrogates it to see if there is a menu
present.

One helper method, `BarState::has_root()`, was added as all the fields
needed to perform the check were private to the `menu` module.

I've also updated the Context Menu demo to show the result of
`is_context_menu_open()` to verify the code functions as expected.

crates/egui/src/context.rs

@@ -1389,6 +1389,14 @@ impl Context {
     pub fn highlight_widget(&self, id: Id) {
         self.frame_state_mut(|fs| fs.highlight_next_frame.insert(id));
     }
+
+    /// Is an egui context menu open?
+    pub fn is_context_menu_open(&self) -> bool {
+        self.data(|d| {
+            d.get_temp::<crate::menu::BarState>(menu::CONTEXT_MENU_ID_STR.into())
+                .map_or(false, |state| state.has_root())
+        })
+    }
 }
 
 // Ergonomic methods to forward some calls often used in 'if let' without holding the borrow

crates/egui/src/menu.rs

@@ -48,6 +48,10 @@ impl BarState {
         MenuRoot::stationary_click_interaction(response, &mut self.open_menu, response.id);
         self.open_menu.show(response, add_contents)
     }
+
+    pub(crate) fn has_root(&self) -> bool {
+        self.open_menu.inner.is_some()
+    }
 }
 
 impl std::ops::Deref for BarState {
@@ -212,12 +216,14 @@ fn stationary_menu_image_impl<'c, R>(
     InnerResponse::new(inner.map(|r| r.inner), button_response)
 }
 
+pub(crate) const CONTEXT_MENU_ID_STR: &str = "__egui::context_menu";
+
 /// Response to secondary clicks (right-clicks) by showing the given menu.
 pub(crate) fn context_menu(
     response: &Response,
     add_contents: impl FnOnce(&mut Ui),
 ) -> Option<InnerResponse<()>> {
-    let menu_id = Id::new("__egui::context_menu");
+    let menu_id = Id::new(CONTEXT_MENU_ID_STR);
     let mut bar_state = BarState::load(&response.ctx, menu_id);
 
     MenuRoot::context_click_interaction(response, &mut bar_state, response.id);

crates/egui_demo_lib/src/demo/context_menu.rs

@@ -66,6 +66,11 @@ impl super::View for ContextMenus {
             ui.menu_button("Click for menu", Self::nested_menus);
             ui.button("Right-click for menu")
                 .context_menu(Self::nested_menus);
+            if ui.ctx().is_context_menu_open() {
+                ui.label("Context menu is open");
+            } else {
+                ui.label("Context menu is closed");
+            }
         });
 
         ui.separator();