Warn if using an TextShape from before a change to pixels_per_point

Closes #1915

epaint/src/shape.rs

@@ -13,6 +13,11 @@ pub use crate::{CubicBezierShape, QuadraticBezierShape};
 
 /// A paint primitive such as a circle or a piece of text.
 /// Coordinates are all screen space points (not physical pixels).
+///
+/// You should generally recreate your [`Shape`]s each frame,
+/// but storing them should also be fine with one exception:
+/// [`Shape::Text`] depends on the current `pixels_per_point` (dpi scale)
+/// and so must be recreated every time `pixels_per_point` changes.
 #[must_use = "Add a Shape to a Painter"]
 #[derive(Clone, Debug, PartialEq)]
 pub enum Shape {
@@ -37,6 +42,8 @@ pub enum Shape {
     Rect(RectShape),
 
     /// Text.
+    ///
+    /// This needs to be recreated if `pixels_per_point` (dpi scale) changes.
     Text(TextShape),
 
     /// A general triangle mesh.
@@ -604,6 +611,8 @@ impl Rounding {
 // ----------------------------------------------------------------------------
 
 /// How to paint some text on screen.
+///
+/// This needs to be recreated if `pixels_per_point` (dpi scale) changes.
 #[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct TextShape {

epaint/src/tessellator.rs

@@ -1334,6 +1334,11 @@ impl Tessellator {
             return;
         }
 
+        if galley.pixels_per_point != self.pixels_per_point {
+            eprintln!("epaint: WARNING: pixels_per_point (dpi scale) have changed between text layout and tessellation. \
+                       You must recreate your text shapes if pixels_per_point changes.");
+        }
+
         out.vertices.reserve(galley.num_vertices);
         out.indices.reserve(galley.num_indices);
 

epaint/src/text/text_layout.rs

@@ -478,6 +478,7 @@ fn galley_from_rows(point_scale: PointScale, job: Arc<LayoutJob>, mut rows: Vec<
         mesh_bounds,
         num_vertices,
         num_indices,
+        pixels_per_point: point_scale.pixels_per_point,
     }
 }
 

epaint/src/text/text_layout_types.rs

@@ -309,6 +309,8 @@ impl Default for TextWrapping {
 /// Text that has been layed out, ready for painting.
 ///
 /// You can create a [`Galley`] using [`crate::Fonts::layout_job`];
+///
+/// This needs to be recreated if `pixels_per_point` (dpi scale) changes.
 #[derive(Clone, Debug, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Galley {
@@ -341,6 +343,12 @@ pub struct Galley {
 
     /// Total number of indices in all the row meshes.
     pub num_indices: usize,
+
+    /// The number of physical pixels for each logical point.
+    /// Since this affects the layout, we keep track of it
+    /// so that we can warn if this has changed once we get to
+    /// tessellation.
+    pub pixels_per_point: f32,
 }
 
 #[derive(Clone, Debug, PartialEq)]