Add Galley::intrinsic_size and use it in AtomLayout (#7146)

- part of https://github.com/emilk/egui/issues/5762
- also allows me to simplify sizing logic in egui_flex

crates/epaint/src/text/text_layout.rs

@@ -82,6 +82,7 @@ pub fn layout(fonts: &mut FontsImpl, job: Arc<LayoutJob>) -> Galley {
             num_indices: 0,
             pixels_per_point: fonts.pixels_per_point(),
             elided: true,
+            intrinsic_size: Vec2::ZERO,
         };
     }
 
@@ -94,6 +95,8 @@ pub fn layout(fonts: &mut FontsImpl, job: Arc<LayoutJob>) -> Galley {
 
     let point_scale = PointScale::new(fonts.pixels_per_point());
 
+    let intrinsic_size = calculate_intrinsic_size(point_scale, &job, &paragraphs);
+
     let mut elided = false;
     let mut rows = rows_from_paragraphs(paragraphs, &job, &mut elided);
     if elided {
@@ -124,7 +127,7 @@ pub fn layout(fonts: &mut FontsImpl, job: Arc<LayoutJob>) -> Galley {
     }
 
     // Calculate the Y positions and tessellate the text:
-    galley_from_rows(point_scale, job, rows, elided)
+    galley_from_rows(point_scale, job, rows, elided, intrinsic_size)
 }
 
 // Ignores the Y coordinate.
@@ -190,6 +193,46 @@ fn layout_section(
     }
 }
 
+/// Calculate the intrinsic size of the text.
+///
+/// The result is eventually passed to `Response::intrinsic_size`.
+/// This works by calculating the size of each `Paragraph` (instead of each `Row`).
+fn calculate_intrinsic_size(
+    point_scale: PointScale,
+    job: &LayoutJob,
+    paragraphs: &[Paragraph],
+) -> Vec2 {
+    let mut intrinsic_size = Vec2::ZERO;
+    for (idx, paragraph) in paragraphs.iter().enumerate() {
+        if paragraph.glyphs.is_empty() {
+            if idx == 0 {
+                intrinsic_size.y += point_scale.round_to_pixel(paragraph.empty_paragraph_height);
+            }
+            continue;
+        }
+        intrinsic_size.x = f32::max(
+            paragraph
+                .glyphs
+                .last()
+                .map(|l| l.max_x())
+                .unwrap_or_default(),
+            intrinsic_size.x,
+        );
+
+        let mut height = paragraph
+            .glyphs
+            .iter()
+            .map(|g| g.line_height)
+            .max_by(|a, b| a.partial_cmp(b).unwrap_or(std::cmp::Ordering::Equal))
+            .unwrap_or(paragraph.empty_paragraph_height);
+        if idx == 0 {
+            height = f32::max(height, job.first_row_min_height);
+        }
+        intrinsic_size.y += point_scale.round_to_pixel(height);
+    }
+    intrinsic_size
+}
+
 // Ignores the Y coordinate.
 fn rows_from_paragraphs(
     paragraphs: Vec<Paragraph>,
@@ -610,6 +653,7 @@ fn galley_from_rows(
     job: Arc<LayoutJob>,
     mut rows: Vec<PlacedRow>,
     elided: bool,
+    intrinsic_size: Vec2,
 ) -> Galley {
     let mut first_row_min_height = job.first_row_min_height;
     let mut cursor_y = 0.0;
@@ -680,6 +724,7 @@ fn galley_from_rows(
         num_vertices,
         num_indices,
         pixels_per_point: point_scale.pixels_per_point,
+        intrinsic_size,
     };
 
     if galley.job.round_output_to_gui {