Add Options::debug_paint_interactive_widgets (#4018)

This paints the rects of all interactive widgets on each layer.


![image](https://github.com/emilk/egui/assets/1148717/3584c4fb-06ce-43d4-bd7c-c5baf927ddf1)

crates/egui/src/context.rs

@@ -1842,6 +1842,27 @@ impl Context {
 
         self.read(|ctx| ctx.plugins.clone()).on_end_frame(self);
 
+        if self.options(|o| o.debug_paint_interactive_widgets) {
+            let rects = self.write(|ctx| ctx.viewport().layer_rects_this_frame.clone());
+            for (layer_id, rects) in rects.by_layer {
+                let painter = Painter::new(self.clone(), layer_id, Rect::EVERYTHING);
+                for rect in rects {
+                    if rect.sense.interactive() {
+                        let (color, text) = if rect.sense.click && rect.sense.drag {
+                            (Color32::from_rgb(0x88, 0, 0x88), "click+drag")
+                        } else if rect.sense.click {
+                            (Color32::from_rgb(0x88, 0, 0), "click")
+                        } else if rect.sense.drag {
+                            (Color32::from_rgb(0, 0, 0x88), "drag")
+                        } else {
+                            (Color32::from_rgb(0, 0, 0x88), "hover")
+                        };
+                        painter.debug_rect(rect.rect, color, text);
+                    }
+                }
+            }
+        }
+
         self.write(|ctx| ctx.end_frame())
     }
 }

crates/egui/src/memory.rs

@@ -214,6 +214,9 @@ pub struct Options {
     ///
     /// By default this is `true` in debug builds.
     pub warn_on_id_clash: bool,
+
+    /// If true, paint all interactive widgets in the order they were added to each layer.
+    pub debug_paint_interactive_widgets: bool,
 }
 
 impl Default for Options {
@@ -227,6 +230,7 @@ impl Default for Options {
             screen_reader: false,
             preload_font_glyphs: true,
             warn_on_id_clash: cfg!(debug_assertions),
+            debug_paint_interactive_widgets: false,
         }
     }
 }
@@ -243,6 +247,7 @@ impl Options {
             screen_reader: _, // needs to come from the integration
             preload_font_glyphs: _,
             warn_on_id_clash,
+            debug_paint_interactive_widgets,
         } = self;
 
         use crate::Widget as _;
@@ -261,6 +266,8 @@ impl Options {
                 );
 
                 ui.checkbox(warn_on_id_clash, "Warn if two widgets have the same Id");
+
+                ui.checkbox(debug_paint_interactive_widgets, "Debug interactive widgets");
             });
 
         use crate::containers::*;