first pass of merging in warp (doesn't build)

crates/galaxyui/src/windowing/winit/windows/clipboard.rs

@@ -2,7 +2,8 @@ use std::ops::Not;
 
 use arboard::{self, Clipboard as WindowsClipboardInner};
 
-use crate::{clipboard::ClipboardContent, Clipboard};
+use crate::clipboard::ClipboardContent;
+use crate::Clipboard;
 
 pub struct WindowsClipboard {
     inner: WindowsClipboardInner,

crates/galaxyui/src/windowing/winit/windows/clipboard_tests.rs

@@ -4,8 +4,9 @@
 /// to avoid duplication. These tests focus on Windows-specific clipboard behavior.
 #[cfg(target_os = "windows")]
 mod clipboard_tests {
+    use crate::clipboard::ClipboardContent;
     use crate::windowing::winit::windows::clipboard::WindowsClipboard;
-    use crate::{clipboard::ClipboardContent, Clipboard};
+    use crate::Clipboard;
 
     fn create_test_clipboard() -> Option<WindowsClipboard> {
         WindowsClipboard::new().ok()

crates/galaxyui/src/windowing/winit/windows/network.rs

@@ -1,4 +1,3 @@
-use crate::windowing::winit::app::CustomEvent;
 use anyhow::Context;
 use windows::core::{implement, Interface};
 use windows::Win32::Networking::NetworkListManager::{
@@ -11,6 +10,8 @@ use windows::Win32::System::Com::{
     COINIT_APARTMENTTHREADED,
 };
 
+use crate::windowing::winit::app::CustomEvent;
+
 /// Implements the INetworkListManagerEvents trait so we can pass along connectivity events from Windows
 /// OS to our winit event loop.
 #[implement(INetworkListManagerEvents)]

crates/galaxyui/src/windowing/winit/windows/registry.rs

@@ -1,7 +1,8 @@
-use crate::platform::SystemTheme;
 use winreg::enums::HKEY_CURRENT_USER;
 use winreg::RegKey;
 
+use crate::platform::SystemTheme;
+
 const SYSTEM_THEME_SUBKEY_PATH: &str =
     "SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
 const LIGHT_MODE_SUBKEY_NAME: &str = "AppsUseLightTheme";

crates/galaxyui/src/windowing/winit/windows/system_caption_buttons.rs

@@ -1,9 +1,10 @@
-use super::window_attribute::get_window_attribute;
-use super::WindowAttributeErr;
 use windows::Win32::Foundation::RECT;
 use windows::Win32::Graphics::Dwm;
 use winit::window::Window as WinitWindow;
 
+use super::window_attribute::get_window_attribute;
+use super::WindowAttributeErr;
+
 #[derive(Debug)]
 pub struct SystemCaptionButtonData {
     bounds: RECT,

crates/galaxyui/src/windowing/winit/windows/window_attribute.rs

@@ -1,10 +1,11 @@
-use std::{ffi::c_void, mem::size_of};
+use std::ffi::c_void;
+use std::mem::size_of;
+
 use thiserror::Error;
 use wgpu::rwh;
 use windows::Win32::Foundation::HWND;
 use windows::Win32::Graphics::Dwm::{self, DWMWINDOWATTRIBUTE};
-use winit::raw_window_handle::HasWindowHandle;
-use winit::raw_window_handle::RawWindowHandle;
+use winit::raw_window_handle::{HasWindowHandle, RawWindowHandle};
 use winit::window::Window as WinitWindow;
 
 #[derive(Debug, Error)]

crates/galaxyui/src/windowing/winit/windows/window_ext.rs

@@ -1,5 +1,9 @@
-use windows::Win32::Foundation::{FALSE, HWND, TRUE};
+use windows::Win32::Foundation::{COLORREF, FALSE, HWND, TRUE};
 use windows::Win32::Graphics::Dwm::{DwmSetWindowAttribute, DWMWA_CLOAK};
+use windows::Win32::UI::WindowsAndMessaging::{
+    GetWindowLongPtrW, SetLayeredWindowAttributes, SetWindowLongPtrW, GWL_EXSTYLE, LWA_ALPHA,
+    WS_EX_LAYERED,
+};
 use windows_core::BOOL;
 use winit::raw_window_handle::{HasWindowHandle, RawWindowHandle};
 use winit::window::Window;
@@ -16,6 +20,13 @@ pub enum Error {
 pub trait WindowExt {
     /// "Cloaks" the window. A cloaked window is one that is invisible, but can still be drawn to.
     fn set_cloaked(&self, cloaked: bool) -> Result<(), Error>;
+
+    /// Sets the window's uniform opacity (`0.0` fully transparent, `1.0` fully
+    /// opaque) using a layered window. Unlike `set_cloaked` or hiding, this keeps
+    /// the window in the z-order and does not change focus, which is what the
+    /// cross-window tab-drag preview relies on while hovering over a target
+    /// window's tab bar.
+    fn set_alpha(&self, alpha: f32) -> Result<(), Error>;
 }
 
 impl WindowExt for Window {
@@ -39,4 +50,32 @@ impl WindowExt for Window {
 
         Ok(())
     }
+
+    fn set_alpha(&self, alpha: f32) -> Result<(), Error> {
+        let Ok(RawWindowHandle::Win32(handle)) = self
+            .window_handle()
+            .map(|window_handle| window_handle.as_raw())
+        else {
+            return Err(Error::InvalidWindowHandle);
+        };
+
+        let hwnd = HWND(handle.hwnd.get() as _);
+        let alpha_byte = (alpha.clamp(0.0, 1.0) * 255.0).round() as u8;
+
+        // SAFETY: `hwnd` is a valid top-level window handle obtained from winit.
+        // `SetLayeredWindowAttributes` requires the `WS_EX_LAYERED` extended
+        // style, so add it if it isn't already present. We intentionally leave
+        // the style set afterwards: a fully-opaque (alpha 255) layered window
+        // composites identically on DWM, which avoids the repaint quirks of
+        // toggling the style off when restoring opacity.
+        unsafe {
+            let ex_style = GetWindowLongPtrW(hwnd, GWL_EXSTYLE);
+            if ex_style & (WS_EX_LAYERED.0 as isize) == 0 {
+                SetWindowLongPtrW(hwnd, GWL_EXSTYLE, ex_style | (WS_EX_LAYERED.0 as isize));
+            }
+            SetLayeredWindowAttributes(hwnd, COLORREF(0), alpha_byte, LWA_ALPHA)?;
+        }
+
+        Ok(())
+    }
 }