Correctly calculate preferred/intrinsic size in AtomLayout

crates/egui/src/atomics/atom_kind.rs

@@ -81,9 +81,21 @@ impl<'a> AtomKind<'a> {
     ) -> (Vec2, SizedAtomKind<'a>) {
         match self {
             AtomKind::Text(text) => {
-                let galley = text.into_galley(ui, wrap_mode, available_size.x, TextStyle::Button);
+                let wrap_mode = wrap_mode.unwrap_or(ui.wrap_mode());
+                let desired_size = matches!(wrap_mode, TextWrapMode::Truncate).then(|| {
+                    text.clone()
+                        .into_galley(
+                            ui,
+                            Some(TextWrapMode::Extend),
+                            available_size.x,
+                            TextStyle::Button,
+                        )
+                        .desired_size()
+                });
+                let galley =
+                    text.into_galley(ui, Some(wrap_mode), available_size.x, TextStyle::Button);
                 (
-                    galley.size(), // TODO(#5762): calculate the preferred size
+                    desired_size.unwrap_or_else(|| galley.desired_size()),
                     SizedAtomKind::Text(galley),
                 )
             }

crates/epaint/src/text/text_layout_types.rs

@@ -795,6 +795,31 @@ impl Galley {
         self.rect.size()
     }
 
+    // TODO: Instead return Option<Vec2>?
+    pub fn desired_size(&self) -> Vec2 {
+        let mut current_width: f32 = 0.0;
+        let mut widest_width: f32 = 0.0;
+        let mut height = self.rows.first().map_or(0.0, |row| row.height());
+        for row in &self.rows {
+            if current_width != 0.0 {
+                let space = row.glyphs.last();
+                if let Some(space) = space {
+                    if space.chr.is_whitespace() {
+                        // TODO: Needed or not? Doesn't seem like it's needed
+                        // current_width += space.advance_width;
+                    }
+                }
+            }
+            current_width += row.rect().width();
+            widest_width = widest_width.max(current_width);
+            if row.ends_with_newline {
+                height += row.height();
+                current_width = 0.0;
+            }
+        }
+        vec2(widest_width, height)
+    }
+
     pub(crate) fn round_output_to_gui(&mut self) {
         for placed_row in &mut self.rows {
             // Optimization: only call `make_mut` if necessary (can cause a deep clone)