Part of #3602: a node thumbnail reports the size it is drawn at

render_node_thumbnail_with draws into a context of size.clamp(1,
MAX_CONTEXT_SIDE) pixels but reported width and height as the requested
size, so a size of 0 gave a 0x0 image holding 4 bytes. The view was
also built from the requested size, and the fit scale (size - 2) /
extent was 0 or negative at sizes 1 and 2, so the art drew nothing or
mirrored.

The thumbnail now reports the side it draws, builds the view from it,
and drops the 1-pixel margin when the side is too small to keep one.

Test: node_thumbnail_reports_the_size_it_draws.

crates/render/src/tests.rs

@@ -869,6 +869,22 @@ fn node_thumbnail_applies_the_root_mask() {
     assert_eq!(img.pixel(75, 50), [255, 255, 255, 255]);
 }
 
+/// #3602: a thumbnail reports the size it was drawn at, and one too small for its 1-pixel margin
+/// still draws the art.
+#[test]
+fn node_thumbnail_reports_the_size_it_draws() {
+    let mut d = Document::new(100.0, 100.0);
+    let node = rect_node(&mut d, Rect::new(10.0, 10.0, 90.0, 90.0), Paint::solid(Color::rgb(1.0, 0.0, 0.0)), Paint::None, 0.0);
+    let mut r = Renderer::new();
+    for size in 0..4 {
+        let img = r.render_node_thumbnail(&d, &node, size, None).unwrap();
+        assert_eq!(img.pixels.len(), img.width as usize * img.height as usize * 4, "size {size}");
+        assert_eq!(img.width, size.max(1), "size {size}");
+        assert_eq!(img.height, size.max(1), "size {size}");
+        assert!(img.pixels.chunks(4).any(|p| p[3] > 0), "size {size}: the art is drawn");
+    }
+}
+
 #[test]
 fn opacity_mask_with_undecodable_image_leaves_out_object() {
     use vectorcraft_doc::{Effect, ImageBlob, ImageObject, NodeKind, OpacityMask};