gpui_windows: Report the global cursor state for drag tracking

Implement Platform::global_cursor_state with GetCursorPos scaled by the
DPI of the monitor under the cursor, and the left button state from
GetAsyncKeyState, so drags released outside every window reach the
tear-off handler on Windows too.

crates/gpui_windows/src/display.rs

@@ -201,7 +201,7 @@ fn generate_uuid(device_name: &[u16]) -> Uuid {
     Uuid::new_v5(&Uuid::NAMESPACE_DNS, &name)
 }
 
-fn get_scale_factor_for_monitor(monitor: HMONITOR) -> Result<f32> {
+pub(crate) fn get_scale_factor_for_monitor(monitor: HMONITOR) -> Result<f32> {
     let mut dpi_x = 0;
     let mut dpi_y = 0;
     unsafe { GetDpiForMonitor(monitor, MDT_EFFECTIVE_DPI, &mut dpi_x, &mut dpi_y) }?;

crates/gpui_windows/src/platform.rs

@@ -636,6 +636,27 @@ impl Platform for WindowsPlatform {
             .map(|inner| inner.handle)
     }
 
+    fn global_cursor_state(&self) -> Option<GlobalCursorState> {
+        let mut point: POINT = unsafe { std::mem::zeroed() };
+        unsafe { GetCursorPos(&mut point) }
+            .context("unable to get cursor position")
+            .log_err()?;
+        let scale_factor = unsafe {
+            let monitor = MonitorFromPoint(point, MONITOR_DEFAULTTONEAREST);
+            if monitor.is_invalid() {
+                1.0
+            } else {
+                get_scale_factor_for_monitor(monitor).log_err()?
+            }
+        };
+        let left_button_pressed =
+            unsafe { GetAsyncKeyState(VK_LBUTTON.0 as i32) } as u16 & 0x8000 != 0;
+        Some(GlobalCursorState {
+            position: logical_point(point.x as f32, point.y as f32, scale_factor),
+            left_button_pressed,
+        })
+    }
+
     fn open_window(
         &self,
         handle: AnyWindowHandle,