diff --git a/config/copy/etc/systemd/user/xdg-document-portal.service.d/panama-stale-mount.conf b/config/copy/etc/systemd/user/xdg-document-portal.service.d/panama-stale-mount.conf
index cbaef50..0a2a34e 100644
--- a/config/copy/etc/systemd/user/xdg-document-portal.service.d/panama-stale-mount.conf
+++ b/config/copy/etc/systemd/user/xdg-document-portal.service.d/panama-stale-mount.conf
@@ -18,5 +18,18 @@
 #
 # `-` because a clean start has nothing to unmount and fusermount3 exits 1
 # saying so, which is not a failure to start on.
+#
+# The unmount alone did not close the hole: the third recurrence (2026-08-26)
+# started on ground the ExecStartPre had cleaned and still came up with no
+# mount, sitting active-and-broken for fourteen hours until the next fresh
+# flatpak launch found it. Hence ExecStartPost: the unit is not "started"
+# until the mount it exists to provide actually exists. Type=dbus declares
+# readiness at bus-name acquisition, which can land before the fuse thread
+# has the mount up, so this polls rather than checks once. If five seconds
+# pass without a mount, the start fails -- loudly, in the journal, as failed
+# -- and the next bus activation retries from the ExecStartPre cleanup
+# instead of every sandbox build failing against a unit that claims to be
+# fine.
 [Service]
 ExecStartPre=-/usr/bin/fusermount3 -u /run/user/%U/doc
+ExecStartPost=/usr/bin/timeout 5 /bin/sh -c 'until /usr/bin/mountpoint -q /run/user/%U/doc; do /usr/bin/sleep 0.2; done'