Hide toolip when opening `ComboBox` drop-down (#4546)

- Fixes #4338


https://github.com/emilk/egui/assets/49431240/73ea87a1-41ad-40b1-b451-d6be2b38c7e0



Tested using `example/hello_world` modified to:
```rust
#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
#![allow(rustdoc::missing_crate_level_docs)] // it's an example

use eframe::egui;

fn main() -> Result<(), eframe::Error> {
    env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
    let options = eframe::NativeOptions {
        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
        ..Default::default()
    };
    eframe::run_native(
        "My egui App",
        options,
        Box::new(|cc| {
            // This gives us image support:
            egui_extras::install_image_loaders(&cc.egui_ctx);

            Box::<MyApp>::default()
        }),
    )
}

struct MyApp {
    name: String,
    age: u32,
}

impl Default for MyApp {
    fn default() -> Self {
        Self {
            name: "Arthur".to_owned(),
            age: 42,
        }
    }
}

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("My egui Application");
            egui::ComboBox::new("combo", "combo box")
                .selected_text(&self.name)
                .show_ui(ui, |ui| {
                    ui.selectable_value(&mut self.name, "Arthur".into(), "Arthur")
                        .on_hover_text("This is Arthur");
                    ui.selectable_value(&mut self.name, "Ford".into(), "Ford")
                        .on_hover_text("This is Ford");
                    ui.selectable_value(&mut self.name, "Trillian".into(), "Trillian")
                        .on_hover_text("This is Trillian");
                })
                .response
                .on_hover_text("This is a combo box");
        });
    }
}
```

---------

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

crates/egui/src/containers/combo_box.rs

@@ -270,6 +270,16 @@ impl ComboBox {
         }
         response
     }
+
+    /// Check if the [`ComboBox`] with the given id has its popup menu currently opened.
+    pub fn is_open(ctx: &Context, id: Id) -> bool {
+        ctx.memory(|m| m.is_popup_open(Self::widget_to_popup_id(id)))
+    }
+
+    /// Convert a [`ComboBox`] id to the id used to store it's popup state.
+    fn widget_to_popup_id(widget_id: Id) -> Id {
+        widget_id.with("popup")
+    }
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -282,7 +292,7 @@ fn combo_box_dyn<'c, R>(
     wrap_mode: Option<TextWrapMode>,
     (width, height): (Option<f32>, Option<f32>),
 ) -> InnerResponse<Option<R>> {
-    let popup_id = button_id.with("popup");
+    let popup_id = ComboBox::widget_to_popup_id(button_id);
 
     let is_popup_open = ui.memory(|m| m.is_popup_open(popup_id));
 

crates/egui/src/response.rs

@@ -2,7 +2,8 @@ use std::{any::Any, sync::Arc};
 
 use crate::{
     emath::{Align, Pos2, Rect, Vec2},
-    menu, Context, CursorIcon, Id, LayerId, PointerButton, Sense, Ui, WidgetRect, WidgetText,
+    menu, ComboBox, Context, CursorIcon, Id, LayerId, PointerButton, Sense, Ui, WidgetRect,
+    WidgetText,
 };
 
 // ----------------------------------------------------------------------------
@@ -571,6 +572,10 @@ impl Response {
             return false;
         }
 
+        if ComboBox::is_open(&self.ctx, self.id) {
+            return false; // Don't cover the open ComboBox with a tooltip
+        }
+
         if self.enabled {
             if !self.hovered || !self.ctx.input(|i| i.pointer.has_pointer()) {
                 return false;