Fix `ui.response().interact(Sense::click())` being flakey (#7713)

This fixes calls to `ui.response().interact(Sense::click())` being
flakey. Since egui checks widget interactions at the beginning of the
frame, based on the responses from last frame, we need to ensure that we
always call `create_widget` on `interact` calls, otherwise there can be
a feedback loop where the `Sense` egui acts on flips back and forth
between frames.

Without the fix in `interact`, both the asserts in the new test fail.

Here is a video where I experienced the bug, showing the sense switching
every frame. Every other click would fail to be detected.


https://github.com/user-attachments/assets/6be7ca0e-b50f-4d30-bf87-bbb80c319f3b

Also note, usually it's better to use `UiBuilder::sense()` to give a Ui
some sense, but sometimes you don't have the flexibility, e.g. in a `Ui`
callback from some code external to your project.

crates/egui/src/response.rs

@@ -724,10 +724,9 @@ impl Response {
     /// ```
     #[must_use]
     pub fn interact(&self, sense: Sense) -> Self {
-        if (self.sense | sense) == self.sense {
-            // Early-out: we already sense everything we need to sense.
-            return self.clone();
-        }
+        // We could check here if the new Sense equals the old one to avoid the extra create_widget
+        // call. But that would break calling `interact` on a response from `Context::read_response`
+        // or `Ui::response`. (See https://github.com/emilk/egui/pull/7713 for more details.)
 
         self.ctx.create_widget(
             WidgetRect {

tests/egui_tests/tests/regression_tests.rs

@@ -1,5 +1,5 @@
 use egui::accesskit::Role;
-use egui::{Align, Color32, Image, Label, Layout, RichText, TextWrapMode, include_image};
+use egui::{Align, Color32, Image, Label, Layout, RichText, Sense, TextWrapMode, include_image};
 use egui_kittest::Harness;
 use egui_kittest::kittest::Queryable as _;
 
@@ -75,3 +75,46 @@ fn combobox_should_have_value() {
         Some("Option 1")
     );
 }
+
+/// This test ensures that `ui.response().interact(...)` works correctly.
+///
+/// This was broken, because there was an optimization in [`egui::Response::interact`]
+/// which caused the [`Sense`] of the original response to flip-flop between `click` and `hover`
+/// between frames.
+///
+/// See <https://github.com/emilk/egui/pull/7713> for more details.
+#[test]
+fn interact_on_ui_response_should_be_stable() {
+    let mut first_frame = true;
+    let mut click_count = 0;
+    let mut harness = Harness::new_ui(|ui| {
+        let ui_response = ui.response();
+        if !first_frame {
+            assert!(
+                ui_response.sense.contains(Sense::click()),
+                "ui.response() didn't have click sense even though we called interact(Sense::click()) last frame"
+            );
+        }
+
+        // Add a label so we have something to click with kittest
+        ui.add(
+            Label::new("senseless label")
+                .sense(Sense::hover())
+                .selectable(false),
+        );
+
+        let click_response = ui_response.interact(Sense::click());
+        if click_response.clicked() {
+            click_count += 1;
+        }
+        first_frame = false;
+    });
+
+    for i in 0..=10 {
+        harness.run_steps(i);
+        harness.get_by_label("senseless label").click();
+    }
+
+    drop(harness);
+    assert_eq!(click_count, 10, "We missed some clicks!");
+}