egui_inspection: PNG-encode screenshots on the wire; collapse protocol feature

- FrameScreenshot now carries PNG bytes instead of raw RGBA (PROTOCOL_VERSION 1→2);
  add a shared `encode_png` helper behind a new `png` feature so the live plugin and the
  kittest harness encode frames identically.
- Make the protocol module unconditional: drop the `protocol` feature flag and the
  optional serde/serde_bytes/rmp-serde deps it gated.
- plugin.rs: re-stamp screenshot-bearing frames with the current step (so inspectors
  waiting for step > prev don't reject them) and pump a tail-side repaint while awaiting
  the GPU readback.

crates/egui_inspection/Cargo.toml

@@ -21,27 +21,27 @@ all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
 
 [features]
-default = ["protocol"]
-
-## Wire-protocol types (`HarnessMessage`, `InspectorCommand`, …) plus length-prefixed
-## MessagePack framing helpers. No `egui` dependency beyond `egui::accesskit`.
-protocol = ["dep:rmp-serde", "dep:serde", "dep:serde_bytes", "egui/serde"]
+default = []
 
 ## Cross-platform local-socket name helpers ([`transport::socket_name`],
 ## [`transport::generate_socket_target`]) built on `interprocess`: unix domain sockets on
 ## unix, named pipes on Windows. Shared by both ends of the connection.
 transport = ["dep:interprocess", "dep:tempfile"]
 
+## [`encode_png`] — shared screenshot PNG encoder so every peer (live plugin + kittest
+## harness) produces identically-encoded frames.
+png = ["dep:image", "image/png"]
+
 ## `InspectionPlugin` — an `egui::Plugin` impl that streams frames + accesskit tree to
 ## an inspector over a local socket and applies received commands. Auto-attaches when
 ## the [`INSPECTION_SOCKET_ENV_VAR`] env var is set.
-plugin = ["protocol", "transport", "dep:image"]
+plugin = ["transport", "png"]
 
 [dependencies]
-egui.workspace = true
-serde = { workspace = true, optional = true }
-serde_bytes = { version = "0.11.17", optional = true }
-rmp-serde = { workspace = true, optional = true }
+egui = { workspace = true, features = ["serde"] }
+serde.workspace = true
+serde_bytes = "0.11.17"
+rmp-serde.workspace = true
 image = { workspace = true, optional = true }
 interprocess = { version = "2.4", optional = true }
 tempfile = { workspace = true, optional = true }

crates/egui_inspection/src/lib.rs

@@ -3,10 +3,8 @@
 //! ## Feature flags
 #![cfg_attr(feature = "document-features", doc = document_features::document_features!())]
 
-#[cfg(feature = "protocol")]
 pub mod protocol;
 
-#[cfg(feature = "protocol")]
 pub use protocol::{
     Capabilities, Frame, FrameScreenshot, HarnessMessage, InspectorCommand, MAX_MESSAGE_BYTES,
     PROTOCOL_VERSION, PeerHello, PeerKind, SourceView, read_message, write_message,
@@ -24,6 +22,12 @@ pub const INSPECTION_SOCKET_ENV_VAR: &str = "EGUI_INSPECTION_SOCKET";
 #[cfg(feature = "transport")]
 pub mod transport;
 
+#[cfg(feature = "png")]
+mod png;
+
+#[cfg(feature = "png")]
+pub use png::encode_png;
+
 #[cfg(feature = "plugin")]
 mod plugin;
 

crates/egui_inspection/src/plugin.rs

@@ -211,11 +211,26 @@ impl egui::Plugin for InspectionPlugin {
                 if let Some(mut frame) = self.pending_frame.take() {
                     let [w, h] = [image.size[0] as u32, image.size[1] as u32];
                     let rgba: Vec<u8> = image.pixels.iter().flat_map(|c| c.to_array()).collect();
-                    frame.screenshot = Some(FrameScreenshot {
-                        width: w,
-                        height: h,
-                        rgba,
-                    });
+                    match crate::encode_png(w, h, &rgba) {
+                        Ok(png) => {
+                            frame.screenshot = Some(FrameScreenshot {
+                                width: w,
+                                height: h,
+                                png,
+                            });
+                        }
+                        Err(err) => {
+                            eprintln!("[INSP] PNG encode failed: {err}");
+                        }
+                    }
+                    // Re-stamp the frame with the *current* step. The stashed `step` was
+                    // captured when we dispatched the screenshot command; in the meantime
+                    // intervening frames (without screenshot) may have been emitted with
+                    // higher step numbers. Inspectors that wait for `step > prev_step` would
+                    // otherwise reject the screenshot-bearing frame because its step has
+                    // regressed.
+                    self.step = self.step.saturating_add(1);
+                    frame.step = self.step;
                     self.send(HarnessMessage::Frame(Box::new(frame)));
                 }
                 break;
@@ -296,6 +311,17 @@ impl egui::Plugin for InspectionPlugin {
         if !want_screenshot {
             // No screenshot needed — emit immediately.
             self.send(HarnessMessage::Frame(Box::new(frame)));
+            // If we're still waiting on a screenshot from a previous dispatch, keep
+            // pumping repaints from the end of the frame too. `input_hook` already
+            // does this at frame start, but on reactive apps the GPU readback can
+            // take several frames to fulfill — without a tail-side repaint the
+            // integration may go idle between `input_hook` ticks once the captured
+            // frame finishes presenting.
+            if self.awaiting_screenshot {
+                if let Some(ctx) = self.shared_ctx.get() {
+                    ctx.request_repaint();
+                }
+            }
             return;
         }
 

crates/egui_inspection/src/png.rs

@@ -0,0 +1,21 @@
+//! Shared screenshot PNG encoder.
+//!
+//! Both peers — the live [`crate::InspectionPlugin`] and `egui_kittest`'s harness inspector
+//! — encode their frames here so they produce identically-encoded
+//! [`crate::protocol::FrameScreenshot`]s.
+
+/// Encode tightly-packed RGBA8 pixels (`width * height * 4` bytes) as PNG using `image`'s
+/// default settings (`CompressionType::Default` + `FilterType::Adaptive`).
+///
+/// PNG keeps high-resolution captures off the hot path of socket throughput — a 1550×2114
+/// RGBA8 buffer is ~13 MiB raw but typically <1 MiB encoded.
+///
+/// # Errors
+/// When the encoder fails (e.g. the buffer length doesn't match `width * height * 4`).
+pub fn encode_png(width: u32, height: u32, rgba: &[u8]) -> Result<Vec<u8>, image::ImageError> {
+    use image::ImageEncoder as _;
+    let mut out = std::io::Cursor::new(Vec::new());
+    image::codecs::png::PngEncoder::new(&mut out)
+        .write_image(rgba, width, height, image::ExtendedColorType::Rgba8)?;
+    Ok(out.into_inner())
+}

crates/egui_inspection/src/protocol.rs

@@ -20,7 +20,7 @@ use egui::accesskit;
 /// non-additive change is made to [`HarnessMessage`] / [`InspectorCommand`] / their
 /// payload structs. The inspector should refuse peers with a higher major version than
 /// it understands.
-pub const PROTOCOL_VERSION: u32 = 1;
+pub const PROTOCOL_VERSION: u32 = 2;
 
 /// What kind of egui peer the inspector is talking to. Determines which controls the
 /// inspector UI should render (Step / Pause buttons make no sense against a live app).
@@ -132,11 +132,13 @@ pub struct FrameScreenshot {
     pub width: u32,
     /// Image height in physical pixels.
     pub height: u32,
-    /// Tightly packed RGBA8 pixels (length = `width * height * 4`). `serde_bytes` encodes
-    /// this as a msgpack `bin` blob (one type tag + raw bytes) instead of the default
-    /// `Vec<u8>` path of one type tag *per byte*, which would roughly double on-wire size.
+    /// PNG-encoded image bytes. PNG compression keeps high-resolution captures off the
+    /// hot path of unix-socket throughput — a 1550×2114 RGBA8 buffer is ~13 MiB raw but
+    /// typically <1 MiB as PNG. `serde_bytes` encodes this as a msgpack `bin` blob (one
+    /// type tag + raw bytes) instead of the default `Vec<u8>` path of one type tag *per
+    /// byte*, which would roughly double on-wire size.
     #[serde(with = "serde_bytes")]
-    pub rgba: Vec<u8>,
+    pub png: Vec<u8>,
 }
 
 /// A single update from the egui peer: accesskit tree + optional screenshot.