Prevent ghost window from showing up on taskbar (#1977)

Add WS_EX_TOOLWINDOW to event target window

CHANGELOG.md

@@ -6,6 +6,7 @@
 - **Breaking:** On Web, remove the `stdweb` backend.
 - Added `Window::focus_window`to bring the window to the front and set input focus.
 - On Wayland and X11, implement `is_maximized` method on `Window`.
+- On Windows, prevent ghost window from showing up in the taskbar after either several hours of use or restarting `explorer.exe`.
 - On macOS, fix issue where `ReceivedCharacter` was not being emitted during some key repeat events.
 - On Wayland, load cursor icons `hand2` and `hand1` for `CursorIcon::Hand`.
 

src/platform_impl/windows/event_loop.rs

@@ -596,7 +596,17 @@ fn create_event_target_window<T: 'static>() -> HWND {
 
     unsafe {
         let window = winuser::CreateWindowExW(
-            winuser::WS_EX_NOACTIVATE | winuser::WS_EX_TRANSPARENT | winuser::WS_EX_LAYERED,
+            winuser::WS_EX_NOACTIVATE
+                | winuser::WS_EX_TRANSPARENT
+                | winuser::WS_EX_LAYERED
+                // WS_EX_TOOLWINDOW prevents this window from ever showing up in the taskbar, which
+                // we want to avoid. If you remove this style, this window won't show up in the
+                // taskbar *initially*, but it can show up at some later point. This can sometimes
+                // happen on its own after several hours have passed, although this has proven
+                // difficult to reproduce. Alternatively, it can be manually triggered by killing
+                // `explorer.exe` and then starting the process back up.
+                // It is unclear why the bug is triggered by waiting for several hours.
+                | winuser::WS_EX_TOOLWINDOW,
             THREAD_EVENT_TARGET_WINDOW_CLASS.as_ptr(),
             ptr::null_mut(),
             0,