eframe: gate web clipboard image copy behind web_clipboard_image feature

The wasm path of `set_clipboard_image` is the only consumer of the
`image` crate inside eframe; `image`'s decoders / color-management code
add ~600 KB compressed to a wasm bundle even when only `png` is
requested (image-webp / moxcms / pxfm get pulled in regardless).

Make the dep optional behind a new `web_clipboard_image` feature
(default-on, so this is source-compatible for everyone except those who
explicitly want the smaller bundle). Apps that opt out get a
`log::warn!` if a CopyImage command ever fires.

crates/eframe/Cargo.toml

@@ -29,12 +29,18 @@ default = [
   "accesskit",
   "default_fonts",
   "wayland", # Required for Linux support (including CI!)
+  "web_clipboard_image",
   "web_screen_reader",
   "wgpu",
   "winit/default",
   "x11",
 ]
 
+## Enable PNG-encoding `egui::ColorImage`s for the web clipboard via the
+## [`image`](https://docs.rs/image) crate. Disable on the web to drop the
+## image (and image-webp / moxcms / ...) crates from the wasm bundle.
+web_clipboard_image = ["dep:image"]
+
 ## Enable platform accessibility API implementations through [AccessKit](https://accesskit.dev/).
 accesskit = ["egui-winit/accesskit"]
 
@@ -195,7 +201,10 @@ windows-sys = { workspace = true, features = [
 # web:
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 bytemuck.workspace = true
-image = { workspace = true, features = ["png"] } # For copying images
+# `image` is only needed for the "copy image to clipboard" path on the web.
+# Behind `web_clipboard_image` (default-on) so apps that don't care can drop
+# the image crate (and ~600 KB of png/webp/moxcms compressed code with it).
+image = { workspace = true, features = ["png"], optional = true }
 js-sys.workspace = true
 percent-encoding.workspace = true
 wasm-bindgen.workspace = true

crates/eframe/src/web/app_runner.rs

@@ -385,7 +385,16 @@ impl AppRunner {
                     super::set_clipboard_text(&text);
                 }
                 egui::OutputCommand::CopyImage(image) => {
+                    #[cfg(feature = "web_clipboard_image")]
                     super::set_clipboard_image(&image);
+                    #[cfg(not(feature = "web_clipboard_image"))]
+                    {
+                        let _ = image;
+                        log::warn!(
+                            "CopyImage requested but eframe was built without the \
+                             `web_clipboard_image` feature; ignoring."
+                        );
+                    }
                 }
                 egui::OutputCommand::OpenUrl(open_url) => {
                     super::open_url(&open_url.url, open_url.new_tab);

crates/eframe/src/web/mod.rs

@@ -207,6 +207,7 @@ fn set_clipboard_text(s: &str) {
 }
 
 /// Set the clipboard image.
+#[cfg(feature = "web_clipboard_image")]
 fn set_clipboard_image(image: &egui::ColorImage) {
     if let Some(window) = web_sys::window() {
         if !window.is_secure_context() {
@@ -250,6 +251,7 @@ fn set_clipboard_image(image: &egui::ColorImage) {
     }
 }
 
+#[cfg(feature = "web_clipboard_image")]
 fn to_image(image: &egui::ColorImage) -> Result<image::RgbaImage, String> {
     profiling::function_scope!();
     image::RgbaImage::from_raw(
@@ -260,6 +262,7 @@ fn to_image(image: &egui::ColorImage) -> Result<image::RgbaImage, String> {
     .ok_or_else(|| "Invalid IconData".to_owned())
 }
 
+#[cfg(feature = "web_clipboard_image")]
 fn to_png_bytes(image: &image::RgbaImage) -> Result<Vec<u8>, String> {
     profiling::function_scope!();
     let mut png_bytes: Vec<u8> = Vec::new();
@@ -272,6 +275,7 @@ fn to_png_bytes(image: &image::RgbaImage) -> Result<Vec<u8>, String> {
     Ok(png_bytes)
 }
 
+#[cfg(feature = "web_clipboard_image")]
 fn create_clipboard_item(mime: &str, bytes: &[u8]) -> Result<web_sys::ClipboardItem, JsValue> {
     let array = js_sys::Uint8Array::from(bytes);
     let blob_parts = js_sys::Array::new();