Add `Context::debug_text` (#3864)

This is a very easy way to show some text under the mouse pointer:

```rs
ctx.debug_text("foo");
```

crates/egui/src/containers/popup.rs

@@ -351,8 +351,6 @@ pub fn popup_above_or_below_widget<R>(
             .fixed_pos(pos)
             .pivot(pivot)
             .show(ui.ctx(), |ui| {
-                // Note: we use a separate clip-rect for this area, so the popup can be outside the parent.
-                // See https://github.com/emilk/egui/issues/825
                 let frame = Frame::popup(ui.style());
                 let frame_margin = frame.total_margin();
                 frame

crates/egui/src/context.rs

@@ -231,6 +231,11 @@ impl ViewportRepaintInfo {
 
 // ----------------------------------------------------------------------------
 
+struct DebugText {
+    location: String,
+    text: WidgetText,
+}
+
 #[derive(Default)]
 struct ContextImpl {
     /// Since we could have multiple viewport across multiple monitors with
@@ -278,6 +283,8 @@ struct ContextImpl {
     accesskit_node_classes: accesskit::NodeClassSet,
 
     loaders: Arc<Loaders>,
+
+    debug_texts: Vec<DebugText>,
 }
 
 impl ContextImpl {
@@ -1060,6 +1067,31 @@ impl Context {
         Self::layer_painter(self, LayerId::debug())
     }
 
+    /// Print this text next to the cursor at the end of the frame.
+    ///
+    /// If you call this multiple times, the text will be appended.
+    ///
+    /// This only works if compiled with `debug_assertions`.
+    ///
+    /// ```
+    /// # let ctx = egui::Context::default();
+    /// # let state = true;
+    /// ctx.debug_text(format!("State: {state:?}"));
+    /// ```
+    #[track_caller]
+    pub fn debug_text(&self, text: impl Into<WidgetText>) {
+        if cfg!(debug_assertions) {
+            let location = std::panic::Location::caller();
+            let location = format!("{}:{}", location.file(), location.line());
+            self.write(|c| {
+                c.debug_texts.push(DebugText {
+                    location,
+                    text: text.into(),
+                });
+            });
+        }
+    }
+
     /// What operating system are we running on?
     ///
     /// When compiling natively, this is
@@ -1578,6 +1610,63 @@ impl Context {
             crate::gui_zoom::zoom_with_keyboard(self);
         }
 
+        let debug_texts = self.write(|ctx| std::mem::take(&mut ctx.debug_texts));
+        if !debug_texts.is_empty() {
+            // Show debug-text next to the cursor.
+            let mut pos = self
+                .input(|i| i.pointer.latest_pos())
+                .unwrap_or_else(|| self.screen_rect().center())
+                + 8.0 * Vec2::Y;
+
+            let painter = self.debug_painter();
+            let where_to_put_background = painter.add(Shape::Noop);
+
+            let mut bounding_rect = Rect::from_points(&[pos]);
+
+            let color = Color32::GRAY;
+            let font_id = FontId::new(10.0, FontFamily::Proportional);
+
+            for DebugText { location, text } in debug_texts {
+                {
+                    // Paint location to left of `pos`:
+                    let location_galley =
+                        self.fonts(|f| f.layout(location, font_id.clone(), color, f32::INFINITY));
+                    let location_rect =
+                        Align2::RIGHT_TOP.anchor_size(pos - 4.0 * Vec2::X, location_galley.size());
+                    painter.galley(location_rect.min, location_galley, color);
+                    bounding_rect = bounding_rect.union(location_rect);
+                }
+
+                {
+                    // Paint `text` to right of `pos`:
+                    let wrap = true;
+                    let available_width = self.screen_rect().max.x - pos.x;
+                    let galley = text.into_galley_impl(
+                        self,
+                        &self.style(),
+                        wrap,
+                        available_width,
+                        font_id.clone().into(),
+                        Align::TOP,
+                    );
+                    let rect = Align2::LEFT_TOP.anchor_size(pos, galley.size());
+                    painter.galley(rect.min, galley, color);
+                    bounding_rect = bounding_rect.union(rect);
+                }
+
+                pos.y = bounding_rect.max.y + 4.0;
+            }
+
+            painter.set(
+                where_to_put_background,
+                Shape::rect_filled(
+                    bounding_rect.expand(4.0),
+                    2.0,
+                    Color32::from_black_alpha(192),
+                ),
+            );
+        }
+
         self.write(|ctx| ctx.end_frame())
     }
 }

crates/egui/src/painter.rs

@@ -219,7 +219,9 @@ impl Painter {
         self.debug_text(pos, Align2::LEFT_TOP, color, format!("🔥 {text}"))
     }
 
-    /// text with a background
+    /// Text with a background.
+    ///
+    /// See also [`Context::debug_text`].
     #[allow(clippy::needless_pass_by_value)]
     pub fn debug_text(
         &self,

crates/egui/src/widget_text.rs

@@ -649,18 +649,39 @@ impl WidgetText {
         fallback_font: impl Into<FontSelection>,
     ) -> Arc<Galley> {
         let wrap = wrap.unwrap_or_else(|| ui.wrap_text());
+        let valign = ui.layout().vertical_align();
+        let style = ui.style();
+
+        self.into_galley_impl(
+            ui.ctx(),
+            style,
+            wrap,
+            available_width,
+            fallback_font.into(),
+            valign,
+        )
+    }
+
+    pub fn into_galley_impl(
+        self,
+        ctx: &crate::Context,
+        style: &Style,
+        wrap: bool,
+        available_width: f32,
+        fallback_font: FontSelection,
+        default_valign: Align,
+    ) -> Arc<Galley> {
         let wrap_width = if wrap { available_width } else { f32::INFINITY };
 
         match self {
             Self::RichText(text) => {
-                let valign = ui.layout().vertical_align();
-                let mut layout_job = text.into_layout_job(ui.style(), fallback_font.into(), valign);
+                let mut layout_job = text.into_layout_job(style, fallback_font, default_valign);
                 layout_job.wrap.max_width = wrap_width;
-                ui.fonts(|f| f.layout_job(layout_job))
+                ctx.fonts(|f| f.layout_job(layout_job))
             }
             Self::LayoutJob(mut job) => {
                 job.wrap.max_width = wrap_width;
-                ui.fonts(|f| f.layout_job(job))
+                ctx.fonts(|f| f.layout_job(job))
             }
             Self::Galley(galley) => galley,
         }