Add mouse-over explanation to duplicate ID warning

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 * `ImageButton` - `ui.add(ImageButton::new(...))`.
 * `ui.vertical_centered` and `ui.vertical_centered_justified`.
+* Mouse-over explanation to duplicate ID warning
 
 ### Changed 🔧
 

egui/src/context.rs

@@ -341,20 +341,34 @@ impl Context {
     /// If the given `Id` is not unique, an error will be printed at the given position.
     /// Call this for `Id`:s that need interaction or persistence.
     pub(crate) fn register_interaction_id(self: &Arc<Self>, id: Id, new_pos: Pos2) {
-        if let Some(prev_pos) = self.memory().used_ids.insert(id, new_pos) {
+        let prev_pos = self.memory().used_ids.insert(id, new_pos);
+        if let Some(prev_pos) = prev_pos {
             if prev_pos.distance(new_pos) < 0.1 {
                 // Likely same Widget being interacted with twice, which is fine.
                 return;
             }
 
+            let show_error = |pos: Pos2, text: String| {
+                let painter = self.debug_painter();
+                let rect = painter.error(pos, text);
+                if let Some(mouse_pos) = self.input.mouse.pos {
+                    if rect.contains(mouse_pos) {
+                        painter.error(
+                            rect.left_bottom() + vec2(2.0, 4.0),
+                            "ID clashes happens when things like Windows or CollpasingHeaders share names,\n\
+                             or when things like ScrollAreas and Resize areas aren't given unique id_source:s.",
+                        );
+                    }
+                }
+            };
+
             let id_str = id.short_debug_format();
 
-            let painter = self.debug_painter();
             if prev_pos.distance(new_pos) < 4.0 {
-                painter.error(new_pos, format!("Double use of ID {}", id_str));
+                show_error(new_pos, format!("Double use of ID {}", id_str));
             } else {
-                painter.error(prev_pos, format!("First use of ID {}", id_str));
-                painter.error(new_pos, format!("Second use of ID {}", id_str));
+                show_error(prev_pos, format!("First use of ID {}", id_str));
+                show_error(new_pos, format!("Second use of ID {}", id_str));
             }
 
             // TODO: a tooltip explaining this.

egui/src/painter.rs

@@ -139,18 +139,20 @@ impl Painter {
         self.text(rect.min, LEFT_TOP, text.into(), text_style, color);
     }
 
-    pub fn error(&self, pos: Pos2, text: impl std::fmt::Display) {
+    pub fn error(&self, pos: Pos2, text: impl std::fmt::Display) -> Rect {
         let text_style = TextStyle::Monospace;
         let font = &self.fonts()[text_style];
         let galley = font.layout_multiline(format!("🔥 {}", text), f32::INFINITY);
         let rect = anchor_rect(Rect::from_min_size(pos, galley.size), LEFT_TOP);
+        let frame_rect = rect.expand(2.0);
         self.add(PaintCmd::Rect {
-            rect: rect.expand(2.0),
+            rect: frame_rect,
             corner_radius: 0.0,
             fill: Srgba::black_alpha(240),
             stroke: Stroke::new(1.0, color::RED),
         });
         self.galley(rect.min, galley, text_style, color::RED);
+        frame_rect
     }
 
     pub fn debug_arrow(&self, origin: Pos2, dir: Vec2, stroke: Stroke) {