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xwayland/selection: use one target window per selection

Simon Ser requested to merge github/fork/Xyene/window-per-selection into master

Created by: Xyene

As before, this PR is based on https://github.com/swaywm/wlroots/pull/2701 and only the last commit contains changes.

Previously, the clipboard and primary selections shared the same window. This was racey, and could have led to pasting failures.

On xfixes selection owner change notification, the logic for requesting the supported mimetypes of the new owner's selection looks like:

xcb_convert_selection(
  xwm->xcb_conn,
  selection->window,
  selection->atom,
  xwm->atoms[TARGETS],
  xwm->atoms[WL_SELECTION],
  selection->timestamp
);

This means: ask the selection owner to write its TARGETS for the selection->atom selection (one of PRIMARY, CLIPBOARD, DND_SELECTION) to selection->window's WL_SELECTION atom.

However, selection->window is shared for both PRIMARY and CLIPBOARD selections, and WL_SELECTION is used as the target atom in both cases. So, there's a race when both selections change at the same time.

The CLIPBOARD selection might support mimetypes {A, B, C}, and the PRIMARY only {A, B}. If the ConvertSelection requests/responses "cross on the wire", so to speak, wlroots can end up believing that the PRIMARY selection also supports C.

A Wayland client may then ask for the PRIMARY selection in C format, which will fail with "convert selection failed". The Wayland client would end up with a failed paste operation.

This commit fixes this by using a separate window for PRIMARY and CLIPBOARD target requests, so that WL_SELECTION can be used as the target atom in both cases.

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