Add better documentation for epi::NativeOptions::drag_and_drop_support

Closes https://github.com/emilk/egui/issues/747

eframe/examples/file_dialog.rs

@@ -95,6 +95,9 @@ impl MyApp {
 }
 
 fn main() {
-    let options = eframe::NativeOptions::default();
+    let options = eframe::NativeOptions {
+        drag_and_drop_support: true,
+        ..Default::default()
+    };
     eframe::run_native(Box::new(MyApp::default()), options);
 }

egui/src/data/input.rs

@@ -60,6 +60,9 @@ pub struct RawInput {
     pub hovered_files: Vec<HoveredFile>,
 
     /// Dragged files dropped into egui.
+    ///
+    /// Note: when using `eframe` on Windows you need to enable
+    /// drag-and-drop support using `epi::NativeOptions`.
     pub dropped_files: Vec<DroppedFile>,
 }
 

egui_demo_app/src/main.rs

@@ -9,6 +9,7 @@ fn main() {
     let options = eframe::NativeOptions {
         // Let's show off that we support transparent windows
         transparent: true,
+        drag_and_drop_support: true,
         ..Default::default()
     };
     eframe::run_native(Box::new(app), options);

epi/src/lib.rs

@@ -180,8 +180,9 @@ pub struct NativeOptions {
     pub decorated: bool,
 
     /// On Windows: enable drag and drop support.
-    /// Default is `false` to avoid issues with crates such as cpal which
-    /// uses that use multi-threaded COM API <https://github.com/rust-windowing/winit/pull/1524>
+    /// Default is `false` to avoid issues with crates such as [`cpal`](https://github.com/RustAudio/cpal) which
+    /// will hang when combined with drag-and-drop.
+    /// See <https://github.com/rust-windowing/winit/issues/1255>.
     pub drag_and_drop_support: bool,
 
     /// The application icon, e.g. in the Windows task bar etc.