Bug 508004

Created attachment 183873 [details]
demo and clipboard selection monitor

**Bug Description**
When an XWayland application attempts to write to the clipboard while KWin currently holds clipboard data provided by a Wayland-native application, a focus change occurring during the write process will often cause the write to fail.
More precisely, the write is overwritten by `kwin_wayland`.

**Reproduction Demo**
I wrote a demo application to reproduce this issue:

* The **main window** monitors and displays the current clipboard content.
* It contains a **button** that opens a popup window for writing to the clipboard.
* Clicking the button opens a window with a text input field and a button, prompting the user to enter some text.
* After entering text, pressing the button writes the text to the clipboard and closes the popup window, causing a focus change.

**Observed Behavior**

* If the clipboard initially contains content copied from a **Wayland-native** application, after performing the above steps, there is a high probability that the main window still shows the old Wayland-provided clipboard content instead of the newly written text.
* If we first use `xclip` to set the clipboard to something written by an **XWayland** application and then repeat the steps, the main window correctly shows the new text every time.

**Additional Investigation**
I also wrote a **clipboard selection monitor**. When the clipboard contains Wayland-provided content:

* The clipboard selection is briefly owned by the demo, but is then immediately re-owned by `kwin_wayland`.

When the clipboard contains XWayland-provided content:

* This re-ownership does not occur.

Looking into KWin’s source code, I found that:

* When KWin receives a **selection change** from an XWayland client, it does not update the selection immediately.
https://github.com/KDE/kwin/blob/e39731feabc643db9c5a6d8c945c29209a047158/src/xwayland/clipboard.cpp#L117

* Instead, it waits until a **selection property change** event is received to obtain a new offer and update the selection.
https://github.com/KDE/kwin/blob/e39731feabc643db9c5a6d8c945c29209a047158/src/xwayland/clipboard.cpp#L133

* Between these two events, other events (e.g., focus changes) can occur.
* If the focus changes before the new offer is obtained, `kwin_wayland` calls `checkWlSource`, which writes the Wayland-side clipboard content back into XWayland, overwriting the new content from the XWayland application.
https://github.com/KDE/kwin/blob/e39731feabc643db9c5a6d8c945c29209a047158/src/xwayland/clipboard.cpp#L73

**Attachments**

1. The demo application described above.
   * Depends on Qt6 Widgets and is built with CMake.

2. The clipboard selection monitor program.
   * Depends on libxcb, libxcb-xfixes, and libxcb-res.
   * Can be built with:

     ```bash
     g++ clipboard_xres_with_targets.cpp -o xcb_clipboard_monitor -lxcb -lxcb-xfixes -lxcb-res
     ```