docker: don't lose a client size that races the exec creation

A browser's terminal-size control frame (NAWS through the console telnet
server) can arrive while client_connected_hook is still creating the
exec; the resize is a no-op then, and the tall default applied after
creation would overwrite it, leaving the session at 511x10000 until the
user resizes.

Record sizes received before the exec exists and prefer them over the
tall default once creation finishes. The recorded size is cleared when
the last client disconnects, together with the restore-to-default.

gns3server/compute/docker/vendor_docker_vm.py

@@ -413,6 +413,7 @@ class _LazyExecTelnetServer(AsyncioTelnetServer):
         self._command = command
         self._allow_resize = allow_resize
         self._exec_id = None
+        self._client_size = None  # size received while no exec existed yet
         self._broadcast_task = None
         self._lock = asyncio.Lock()
         self._log_name = f"docker_exec console '{vm.name}'"
@@ -436,18 +437,24 @@ class _LazyExecTelnetServer(AsyncioTelnetServer):
         # inherit it and hit PTY-window paging (the IOS-XR --More-- trap).
         if self._exec_id and not await self._get_connections_snapshot():
             with contextlib.suppress(Exception):
+                self._client_size = None
                 await self._resize_exec(511, 10000)
 
     async def _resize_exec(self, columns, rows):
-        if self._exec_id:
-            try:
-                await self._manager.query(
-                    "POST",
-                    f"exec/{self._exec_id}/resize",
-                    params={"h": str(rows), "w": str(columns)},
-                )
-            except DockerError:
-                pass
+        if not self._exec_id:
+            # No exec yet (first client still inside client_connected_hook):
+            # remember the size — the hook applies it right after creation
+            # instead of the tall default, so it doesn't get overwritten.
+            self._client_size = (columns, rows)
+            return
+        try:
+            await self._manager.query(
+                "POST",
+                f"exec/{self._exec_id}/resize",
+                params={"h": str(rows), "w": str(columns)},
+            )
+        except DockerError:
+            pass
 
     async def _on_naws(self, columns, rows):
         # Client-driven resize (WS terminal-size control frames, telnet NAWS).
@@ -544,10 +551,13 @@ class _LazyExecTelnetServer(AsyncioTelnetServer):
                     # (e.g. the IOS-XR pager) park at --More-- for clients
                     # that never negotiate NAWS (netmiko, bare telnet).
                     # Width 511 matches netmiko's 'terminal width 511'.
-                    # WebUI clients resize to their real geometry right after
-                    # connecting, via WS terminal-size control frames turned
-                    # into NAWS by start_websocket_console.
-                    await self._resize_exec(511, 10000)
+                    # A size already pushed by this client (WS terminal-size
+                    # control frames -> NAWS, racing the exec creation) wins
+                    # over the default.
+                    if self._client_size:
+                        await self._resize_exec(*self._client_size)
+                    else:
+                        await self._resize_exec(511, 10000)
                 except Exception:
                     pass
             else: