Fix buggy interaction with widgets outside of clip rect (#4675)

This fixes a bug which sometimes would make it possible to interact with
widgets that were outside the parent clip_rect.

Interaction with a widget is done with the `interact_rect`, which is the
intersection of the widget rect and the parent clip rect. If these
rectangles are disjoint (the widget is outside the parent clip rect),
this results in a _negative rectangle_ (a rectangle with a negative
width and/or height). The distance tests for negative rectangles were
broken, causing the bug.

* This is part of solving https://github.com/emilk/egui/issues/4475
* It is also likely this would have solved
https://github.com/emilk/egui/issues/4349 (which now has another fix for
it)


### Breaking changes
`Rect::distance_to_pos`, `distance_sq_to_pos`, `signed_distance_to_pos`
now all return `f32::INFINITY` if the rectangle is negative.

crates/egui/src/hit_test.rs

@@ -58,6 +58,10 @@ pub fn hit_test(
         .filter(|layer| layer.order.allow_interaction())
         .flat_map(|&layer_id| widgets.get_layer(layer_id))
         .filter(|&w| {
+            if w.interact_rect.is_negative() {
+                return false;
+            }
+
             let pos_in_layer = pos_in_layers.get(&w.layer_id).copied().unwrap_or(pos);
             let dist_sq = w.interact_rect.distance_sq_to_pos(pos_in_layer);
 
@@ -311,6 +315,10 @@ fn find_closest(widgets: impl Iterator<Item = WidgetRect>, pos: Pos2) -> Option<
     let mut closest = None;
     let mut closest_dist_sq = f32::INFINITY;
     for widget in widgets {
+        if widget.interact_rect.is_negative() {
+            continue;
+        }
+
         let dist_sq = widget.interact_rect.distance_sq_to_pos(pos);
 
         // In case of a tie, take the last one = the one on top.

crates/emath/src/rect.rs

@@ -361,6 +361,8 @@ impl Rect {
     /// The distance from the rect to the position.
     ///
     /// The distance is zero when the position is in the interior of the rectangle.
+    ///
+    /// [Negative rectangles](Self::is_negative) always return [`f32::INFINITY`].
     #[inline]
     pub fn distance_to_pos(&self, pos: Pos2) -> f32 {
         self.distance_sq_to_pos(pos).sqrt()
@@ -369,8 +371,14 @@ impl Rect {
     /// The distance from the rect to the position, squared.
     ///
     /// The distance is zero when the position is in the interior of the rectangle.
+    ///
+    /// [Negative rectangles](Self::is_negative) always return [`f32::INFINITY`].
     #[inline]
     pub fn distance_sq_to_pos(&self, pos: Pos2) -> f32 {
+        if self.is_negative() {
+            return f32::INFINITY;
+        }
+
         let dx = if self.min.x > pos.x {
             self.min.x - pos.x
         } else if pos.x > self.max.x {
@@ -394,6 +402,8 @@ impl Rect {
     ///
     /// Negative inside the box.
     ///
+    /// [Negative rectangles](Self::is_negative) always return [`f32::INFINITY`].
+    ///
     /// ```
     /// # use emath::{pos2, Rect};
     /// let rect = Rect::from_min_max(pos2(0.0, 0.0), pos2(1.0, 1.0));
@@ -402,6 +412,10 @@ impl Rect {
     /// assert_eq!(rect.signed_distance_to_pos(pos2(1.50, 0.50)), 0.50);
     /// ```
     pub fn signed_distance_to_pos(&self, pos: Pos2) -> f32 {
+        if self.is_negative() {
+            return f32::INFINITY;
+        }
+
         let edge_distances = (pos - self.center()).abs() - self.size() * 0.5;
         let inside_dist = edge_distances.max_elem().min(0.0);
         let outside_dist = edge_distances.max(Vec2::ZERO).length();