Our internal state machine gets screwed up if selection events are not
monotonically increasing in time, and we can enter a self-copy loop from
the proxy window that exhausts all pipes.
Snippet of logs when this occurs:
00:00:46.238 [wlr] [xwayland/selection/incoming.c:487] XCB_XFIXES_SELECTION_NOTIFY (selection=277, owner=4194626)
00:00:46.238 [wlr] [xwayland/selection/incoming.c:487] XCB_XFIXES_SELECTION_NOTIFY (selection=277, owner=2097153)
00:00:46.238 [wlr] [xwayland/selection/outgoing.c:378] XCB_SELECTION_REQUEST (time=58979563 owner=2097153, requestor=2097153 selection=277, target=279, property=278)
00:00:46.238 [wlr] [xwayland/selection/outgoing.c:397] ignoring old request from timestamp 58979563; expecting > 58979563
00:00:46.238 [wlr] [xwayland/selection/outgoing.c:29] SendEvent destination=2097153 SelectionNotify(31) time=58979563 requestor=2097153 selection=277 target=279 property=0
00:00:46.238 [wlr] [xwayland/selection/incoming.c:453] XCB_SELECTION_NOTIFY (selection=277, property=0, target=279)
Note that 2097153 is `selection->window`, and 4194626 is Emacs.
The race occurs if the selection owner changes back to our proxy window
between when we get `XCB_XFIXES_SELECTION_NOTIFY` for Emacs and when we
call `xcb_convert_selection` in `incoming.c:source_send` -- the
ConvertSelection request can end up hitting our proxy window, but the
timestamp will be rejected.
Fixes#2192.
7d52b4d0b5