From 61bc027072e5179df64687a4957c82b66c0b6324 Mon Sep 17 00:00:00 2001
From: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>
Date: Thu, 15 Oct 2026 11:56:32 +0200
Subject: [PATCH] Fix MCP layer screenshots exceeding maxSize

Fixes #445

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 contributors/people.toml                   |  4 ++
 crates/mcp/src/tools.rs                    | 15 +++++-
 crates/mcp/tests/screenshot_pixel_limit.rs | 53 ++++++++++++++++++++++
 3 files changed, 70 insertions(+), 2 deletions(-)
 create mode 100644 crates/mcp/tests/screenshot_pixel_limit.rs

diff --git a/contributors/people.toml b/contributors/people.toml
index e61cfd31..db1f2b54 100644
--- a/contributors/people.toml
+++ b/contributors/people.toml
@@ -16,3 +16,7 @@
 #
 # To remove your names, delete your entry (or set fields empty) in a PR. The next
 # contributors.py run, and the next build, drop them.
+
+[people.Silvertree2010]
+display_name = "Netter"
+sync_github_name = false
diff --git a/crates/mcp/src/tools.rs b/crates/mcp/src/tools.rs
index b8d3f3ad..389d75de 100644
--- a/crates/mcp/src/tools.rs
+++ b/crates/mcp/src/tools.rs
@@ -721,8 +721,19 @@ fn screenshot(b: &mut dyn Backend, a: &Map<String, Value>) -> Result<ToolResult,
         let side = (x1 - x0).max(y1 - y0).max(1e-6);
         scale = scale.min(max / side);
     }
-    let bytes = b.render_png(&ids, scale)?;
-    let (w, h) = png_info(&bytes);
+    let mut bytes = b.render_png(&ids, scale)?;
+    let (mut w, mut h) = png_info(&bytes);
+    // Layer bounds leave out effects (shadows) and overflow of unclipped frames, but the render
+    // includes them: shrink to the measured image until it fits `maxSize` (rounding can add a pixel).
+    for _ in 0..4 {
+        let side = f64::from(w.max(h));
+        if side <= max {
+            break;
+        }
+        scale *= max / side;
+        bytes = b.render_png(&ids, scale)?;
+        (w, h) = png_info(&bytes);
+    }
     Ok(ToolResult::image(
         &bytes,
         "image/png",
diff --git a/crates/mcp/tests/screenshot_pixel_limit.rs b/crates/mcp/tests/screenshot_pixel_limit.rs
new file mode 100644
index 00000000..095992ff
--- /dev/null
+++ b/crates/mcp/tests/screenshot_pixel_limit.rs
@@ -0,0 +1,53 @@
+//! `screenshot` with `ids` must honour `maxSize` for the rendered paint area (shadows, overflow).
+#![allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
+mod common;
+use common::{Client, png_dims};
+use serde_json::{Value, json};
+
+fn dimensions(client: &mut Client, args: Value) -> (u32, u32) {
+    let result = client.call("screenshot", args);
+    assert_eq!(result["isError"], false, "{result}");
+    let size = png_dims(result["content"][0]["data"].as_str().unwrap());
+    let meta: Value = serde_json::from_str(result["content"][1]["text"].as_str().unwrap()).unwrap();
+    assert_eq!(meta["width"], size.0, "{meta}");
+    assert_eq!(meta["height"], size.1, "{meta}");
+    size
+}
+
+#[test]
+fn layer_screenshots_fit_the_requested_size() {
+    let mut shadow_client = Client::new();
+    let shadow = shadow_client.create(json!({
+        "type": "rectangle", "x": 0, "y": 0,
+        "width": 100, "height": 100,
+        "props": {"effects": [{
+            "type": "dropShadow", "color": "#00000080",
+            "offset": [0, 40], "radius": 60
+        }]}
+    }));
+    let shadow_size = dimensions(&mut shadow_client, json!({"ids": [shadow], "maxSize": 100}));
+
+    let mut frame_client = Client::new();
+    let frame = frame_client.create(json!({
+        "type": "frame", "x": 0, "y": 0,
+        "width": 100, "height": 100, "props": {"clip": false}
+    }));
+    frame_client.create(json!({
+        "type": "rectangle", "parent": frame,
+        "x": 900, "y": 900, "width": 100, "height": 100
+    }));
+    let frame_size = dimensions(&mut frame_client, json!({"ids": [frame], "maxSize": 100}));
+
+    assert!(shadow_size.0 <= 100 && shadow_size.1 <= 100, "shadow screenshot exceeds maxSize: {shadow_size:?}");
+    assert!(shadow_size.0 >= 90 && shadow_size.1 >= 90, "shadow screenshot shrank too far: {shadow_size:?}");
+    assert!(frame_size.0 <= 100 && frame_size.1 <= 100, "frame screenshot exceeds maxSize: {frame_size:?}");
+    assert!(frame_size.0 >= 90 && frame_size.1 >= 90, "frame screenshot shrank too far: {frame_size:?}");
+}
+
+#[test]
+fn plain_layer_screenshot_sizes_are_unchanged() {
+    let mut client = Client::new();
+    let rect = client.create(json!({"type": "rectangle", "x": 0, "y": 0, "width": 100, "height": 100}));
+    assert_eq!(dimensions(&mut client, json!({"ids": [rect], "maxSize": 100})), (100, 100));
+    assert_eq!(dimensions(&mut client, json!({"ids": [rect], "scale": 2})), (200, 200));
+}