handwritin: Grow and shrink the canvas without refreshing

src/handwriting/canvas_rasterizer.rs

@@ -18,6 +18,7 @@ const CHUNK_SIZE: usize = 64;
 /// Rasterize onto a resizeable canvas.
 #[derive(Default)]
 pub struct CanvasRasterizer {
+    image_size: [usize; 2],
     tiles: HashMap<[usize; 2], Tile>,
 }
 
@@ -49,12 +50,30 @@ impl Tile {
 }
 
 impl CanvasRasterizer {
-    //pub fn clear(&mut self) {
-    //    self.image = ColorImage::new(self.image.size, Color32::TRANSPARENT);
-    //    self.texture_is_dirty = true;
-    //}
-
     pub fn set_size(&mut self, width: usize, height: usize) {
+        self.image_size = [width, height];
+        self.populate_tiles();
+    }
+
+    pub fn clear(&mut self) {
+        log::error!("clearing all tiles");
+        self.tiles.clear();
+        self.populate_tiles();
+    }
+
+    pub fn clear_and_set_size(&mut self, width: usize, height: usize) {
+        self.image_size = [width, height];
+        self.clear();
+    }
+
+    fn populate_tiles(&mut self) {
+        let [width, height] = self.image_size;
+
+        // discard tiles that are out of bounds
+        self.tiles.retain(|_, tile| {
+            tile.bounding_box.x_from <= width && tile.bounding_box.y_from <= height
+        });
+
         let chunk = |max: usize| {
             (0..)
                 .step_by(CHUNK_SIZE)
@@ -62,6 +81,7 @@ impl CanvasRasterizer {
                 .enumerate()
         };
 
+        // create new tiles where we need them
         for (xi, _x) in chunk(width) {
             for (yi, _y) in chunk(height) {
                 self.tiles
@@ -71,13 +91,6 @@ impl CanvasRasterizer {
         }
     }
 
-    pub fn clear_and_set_size(&mut self, width: usize, height: usize) {
-        log::error!("clearing all tiles");
-        self.tiles.clear();
-
-        self.set_size(width, height);
-    }
-
     pub fn rasterize<'a>(
         &mut self,
         point_to_pixel: TSTransform,