Fix permanent input-timing desync; no i-frames; UI respawn; guard dead joins

The real cause of the ship/bullet separation, which the previous commit only
half-addressed. The server dropped inputs past a lead of 12 while the client
only re-synced past 16, so a client whose lead drifted into 13-16 had every
input silently rejected while believing its timing was fine. The server coasted
on held_input and then stopped; the client kept predicting. The two separated
permanently and the reconciler fought it every snapshot -- "shoved around".
It needed two independent clocks to drift, hence "only after some time", and
nothing in the loop could notice, hence "then persists". The listen-server
diagnostic could never reproduce it: one process, one physics tick, lead
constant by construction.

Two defences: INPUT_MAX_LEAD (40) is now far wider than the client's correction
band (3..20), asserted by tests/unit/test_input_lead.gd so narrowing it fails a
test; and an ack-stall detector re-syncs when last_input_tick stops advancing,
which catches the whole class regardless of cause -- lead alone cannot, because
a wrong lead looks normal from the client. diag_prediction.gd now injects a +14
tick drift and exits non-zero unless the gap recovers.

Also:
- No invulnerability frames. Every bullet that touches a player lands; i-frames
  made dense patterns safer than sparse ones, which inverts the genre. Measured:
  a stationary player survives ~13.6s of the Warden's opening phase, ~17.5s
  drifting. spawn_grace remains the only invulnerable state.
- Death is exited with a HUD button, disabled for the first 3s. The lockout is
  enforced in SimWorld, not just by graying the button -- a client that ignores
  its own UI still waits. The interact key no longer respawns.
- Joining a server that is not there no longer drops the player into an empty
  lobby they cannot act in. Net.join() only creates an ENet object; the game
  scene now waits for the server to actually place us in an instance, with an
  8s timeout, and headless runs exit non-zero instead of idling.

Protocol 2 -> 3. 98 tests; check.sh, test.sh and smoke.sh all pass.

CLAUDE.md

@@ -110,6 +110,15 @@ ticks in milliseconds with no SceneTree.
 - **`PLAYER_RADIUS` (hitbox) < `PLAYER_VISUAL_RADIUS` (sprite), and
   `PLAYER_MUZZLE_OFFSET` derives from the visual one.** Prefer a visible
   near-miss over an invisible hit; keep the muzzle clear of the sprite.
+- **`INPUT_MAX_LEAD` must stay well above `INPUT_LEAD_MAX`.** The server's
+  input acceptance window has to be wider than the band in which the client
+  re-syncs its own numbering. Violate it and drifting clocks land in a silent
+  dead zone where the server rejects everything and the client never notices —
+  the ship and the authoritative position separate permanently. Pinned by
+  `tests/unit/test_input_lead.gd`.
+- **No i-frames.** Every bullet that touches a player lands; `spawn_grace` is
+  the only invulnerable state. Do not reintroduce post-hit immunity — it makes
+  dense patterns safer than sparse ones.
 - **A disconnect is not an exit.** Dropping in a dungeon keeps the player in the
   world as `linkdead`, channelling out over the same second the escape costs.
   Damage must never cancel the escape channel, or quitting beats the button.

docs/NETCODE.md

@@ -71,6 +71,50 @@ slow relative to bullet speed, so this reads as latency rather than unfairness.
 If it becomes a complaint, the fix is to lag-compensate **player bullets against
 enemies only**, never enemy bullets against players.
 
+## Input timing, and a bug worth remembering
+
+The client numbers its input frames `INPUT_TARGET_LEAD` ticks ahead of the
+server's tick; the server accepts anything within `INPUT_MAX_LEAD` and re-syncs
+nothing itself. **These two numbers must be chosen together.**
+
+They originally were not — the server dropped inputs past a lead of 12, while
+the client only corrected itself past 16. A client whose lead drifted into 13–16
+therefore had *every* input silently rejected, while believing its own timing
+was fine. The server, receiving nothing, coasted on the last input it had
+(`held_input`) and then stopped; the client kept predicting forward. The ship
+and the authoritative position separated permanently, the reconciler fought it
+every snapshot, and the player got shoved around. It needed two independent
+clocks to drift apart, so it appeared only after minutes of play — and never
+recovered, because nothing in the loop could notice.
+
+Two defences now:
+
+1. `INPUT_MAX_LEAD` (40) is far wider than the client's correction band
+   (`INPUT_LEAD_MIN` 3 … `INPUT_LEAD_MAX` 20), so the client always re-syncs
+   long before the server starts refusing anything. `test_input_lead.gd`
+   asserts that ordering directly, so narrowing the window fails a test rather
+   than shipping.
+2. An **ack-stall detector**: if `last_input_tick` does not advance across
+   `INPUT_ACK_STALL_LIMIT` snapshots, the client concludes its numbering is
+   outside the window and hard re-syncs. Lead alone cannot detect this, because
+   a wrong lead looks perfectly normal from the client's side. This is the
+   backstop that makes the whole class of failure self-healing regardless of
+   cause.
+
+`tools/diag_prediction.gd` injects a +14 tick drift mid-run and asserts the gap
+returns to normal; it exits non-zero if it does not.
+
+## No invulnerability frames
+
+A hit grants no immunity — every bullet that touches you deals its damage. In a
+bullet hell the wall *is* the threat, and i-frames invert that: you are punished
+for the first bullet of a pattern and gifted the next thirty, which makes dense
+patterns *safer* than sparse ones. Measured cost: a stationary player survives
+~13.6s of the Warden's opening phase, ~17.5s while drifting aimlessly.
+
+Arrival protection (`SPAWN_GRACE_TICKS`) is the sole exception, and it is a
+transition, not a combat mechanic.
+
 ## Input validation
 
 `SimWorld.queue_input()` is the single audit point. It drops:
@@ -81,6 +125,11 @@ enemies only**, never enemy bullets against players.
 | Stale input | `f.tick < tick - INPUT_MAX_AGE` |
 | Input claiming the future | `f.tick > tick + INPUT_MAX_LEAD` |
 | Flood | queue capped at `INPUT_MAX_AGE`, oldest dropped |
+| Leaving the hub early after death | `RESPAWN_LOCKOUT_TICKS`, server-side |
+
+The respawn lockout is worth calling out: the HUD disables its button for the
+same three seconds, but that is presentation. A client that ignores its own UI
+and holds the bit down still waits, because the check lives in `SimWorld`.
 
 Beyond that, the wire format itself constrains the cheat surface: the move
 vector is two signed bytes at 1/100 precision, and `Movement.sanitize_move()`

src/core/sim_config.gd

@@ -42,19 +42,46 @@ const PLAYER_BULLET_SPEED := 620.0
 const PLAYER_BULLET_RADIUS := 4.0
 const PLAYER_BULLET_LIFETIME := 90       # ticks
 const PLAYER_BULLET_DAMAGE := 6
-const PLAYER_IFRAMES := 36               # ticks of invulnerability after a hit
+## There are deliberately NO invulnerability frames after a hit. In a bullet
+## hell the wall of bullets IS the threat, and i-frames turn a dense pattern
+## into a single cheap hit -- you get punished for the first bullet and gifted
+## the next thirty. Every bullet that touches you lands. Arrival protection
+## below is the one exception, and it is a transition, not a combat mechanic.
+
 ## Invulnerable *and* unable to shoot on entering a dungeon, so arriving into a
 ## live bullet field is survivable. Both halves matter: invulnerability alone
 ## would make the spawn point a free firing position.
 const SPAWN_GRACE_TICKS := 120           # 2 seconds
+## A downed player cannot leave for the hub until this has elapsed. Enforced
+## here rather than only by grey-ing out the button, because a disabled button
+## is a suggestion -- the server is the only thing a modified client cannot
+## argue with.
+const RESPAWN_LOCKOUT_TICKS := 180       # 3 seconds
 
-# --- Anti-cheat guards ------------------------------------------------------
+# --- Input timing -----------------------------------------------------------
+# The client numbers its input frames INPUT_TARGET_LEAD ahead of the server's
+# tick and re-syncs whenever its lead leaves [INPUT_LEAD_MIN, INPUT_LEAD_MAX].
+# The server accepts anything within INPUT_MAX_LEAD.
+#
+# INPUT_MAX_LEAD *must* stay comfortably above INPUT_LEAD_MAX. When it did not
+# (12 vs 16), a client whose lead drifted into the gap had every input silently
+# rejected while believing its timing was fine -- so the server coasted on a
+# stale input and the player's ship and the authoritative position separated
+# permanently. Two independent clocks have to drift for it to happen, which is
+# why it only showed up minutes in, and never recovered.
 ## Inputs older than this (relative to the newest accepted) are discarded.
 const INPUT_MAX_AGE := 30
-## Inputs claiming to be further ahead than this of the server tick are clamped.
+## Where the client aims to sit.
-const INPUT_MAX_LEAD := 12
+const INPUT_TARGET_LEAD := 8
-## Hard ceiling on inputs consumed from one peer in a single tick.
+## The client re-syncs its numbering outside this band.
-const INPUT_MAX_PER_TICK := 4
+const INPUT_LEAD_MIN := 3
+const INPUT_LEAD_MAX := 20
+## Inputs claiming to be further ahead than this of the server tick are dropped.
+const INPUT_MAX_LEAD := 40
+## Snapshots with no forward progress on last_input_tick before the client
+## assumes its numbering is out of the server's window and hard re-syncs. The
+## backstop that makes the failure above self-healing whatever its cause.
+const INPUT_ACK_STALL_LIMIT := 8
 
 # --- Emergency escape -------------------------------------------------------
 const ESCAPE_CHANNEL_TICKS := 60         # 1 second

src/main.gd

@@ -5,9 +5,15 @@ extends Node
 
 const GAME_SCENE := preload("res://src/game.tscn")
 
+## How long to wait for a server to answer before giving up. ENet's own
+## connection_failed can take considerably longer than a player's patience.
+const CONNECT_TIMEOUT_SEC := 8.0
+
 var _menu: Control = null
 var _game: Node = null
 var _ticks: int = 0
+var _connecting: bool = false
+var _connect_deadline: float = 0.0
 
 
 func _ready() -> void:
@@ -30,6 +36,8 @@ func _ready() -> void:
 
 
 func _physics_process(_delta: float) -> void:
+	if _connecting and _now() > _connect_deadline:
+		_abort_connect("no answer from %s:%d" % [GameOpts.host_address, GameOpts.port])
 	_ticks += 1
 	if GameOpts.autoquit_ticks > 0 and _ticks >= GameOpts.autoquit_ticks:
 		GameLog.info("main", "autoquit after %d ticks" % _ticks)
@@ -59,15 +67,49 @@ func _show_menu() -> void:
 	add_child(_menu)
 
 
+## Note what this deliberately does NOT do: enter the game. Net.join() only
+## means "an ENet client object was created" -- it says nothing about anything
+## being at the other end. Showing the world at that point dropped the player
+## into an empty lobby they could not act in, because no server had ever
+## acknowledged them. The game scene now waits for the server to actually place
+## us in an instance.
 func _join(address: String, port: int) -> void:
 	GameOpts.host_address = address
 	GameOpts.port = port
 	if Net.join(address, port) != OK:
 		_menu_status(Net.last_error)
 		return
+	_connecting = true
+	_connect_deadline = _now() + CONNECT_TIMEOUT_SEC
+	_menu_busy(true)
+	_menu_status("connecting to %s:%d ..." % [address, port])
+	Net.client.instance_changed.connect(_on_session_ready, CONNECT_ONE_SHOT)
+
+
+func _now() -> float:
+	return float(Time.get_ticks_msec()) / 1000.0
+
+
+## The server has accepted the handshake and placed us somewhere. Only now is
+## there a world worth drawing.
+func _on_session_ready() -> void:
+	_connecting = false
 	_enter_game()
 
 
+func _abort_connect(reason: String) -> void:
+	_connecting = false
+	GameLog.error("main", "connection failed: %s" % reason)
+	if GameOpts.autojoin:
+		# Headless/scripted run: there is no menu to fall back to, and silently
+		# idling would make a broken smoke test look like a passing one.
+		get_tree().quit(1)
+		return
+	Net.shutdown()
+	_show_menu()
+	_menu_status(reason)
+
+
 ## Listen server: one process runs the authoritative world and a local client
 ## connected over loopback. The client takes no shortcuts because of it -- it
 ## still only sends input and still only learns outcomes from snapshots.
@@ -100,9 +142,17 @@ func _menu_status(text: String) -> void:
 		_menu.set_status(text)
 
 
+func _menu_busy(busy: bool) -> void:
+	if _menu != null:
+		_menu.set_busy(busy)
+
+
 func _on_net_state(state: Net.State) -> void:
 	match state:
 		Net.State.FAILED:
+			if _connecting:
+				_abort_connect(Net.last_error)
+				return
 			_show_menu()
 			_menu_status(Net.last_error)
 		Net.State.OFFLINE:

src/net/client_runtime.gd

@@ -35,6 +35,13 @@ var my_escape: float = 0.0
 var my_escaping: bool = false
 ## Arrival protection: invulnerable and unable to shoot.
 var my_spawn_grace: bool = false
+## Seconds left before the HUD's return-to-hub button becomes available. The
+## server enforces the same lockout; this only drives the button's look.
+var my_respawn_wait: float = 0.0
+
+## Set by the HUD button, not by a key. Death is deliberately exited through a
+## deliberate click rather than whatever the player happened to be holding.
+var request_respawn: bool = false
 
 ## Set by the in-game menu's "return to hub" -- synthesises a held escape
 ## button, so the menu route runs the identical server-side channel as the key.
@@ -48,6 +55,12 @@ var roster: Array[Dictionary] = []
 ## Protocol.COUNTDOWN_NONE outside that state.
 var cleared_countdown: int = Protocol.COUNTDOWN_NONE
 
+## Backstop for input-numbering drift: if the server stops acknowledging new
+## inputs, our tick numbering has fallen outside its acceptance window and no
+## amount of waiting fixes it. Counted in snapshots, not ticks.
+var _last_acked: int = -1
+var _ack_stall: int = 0
+
 var snap_prev: Dictionary = {}
 var snap_curr: Dictionary = {}
 var _interp: float = 0.0
@@ -98,6 +111,11 @@ func _physics_process(delta: float) -> void:
 func _sample_input() -> InputFrame:
 	if GameOpts.bot_client:
 		return _bot_input()
+	if not my_alive:
+		# Downed: no movement, no fire, and the interact key does nothing. The
+		# only way out is the HUD button, which sets request_respawn.
+		var dead_buttons := InputFrame.BTN_INTERACT if request_respawn else 0
+		return InputFrame.make(input_tick, Vector2.ZERO, aim, dead_buttons)
 	var move := Input.get_vector("move_left", "move_right", "move_up", "move_down")
 	var mouse := get_viewport().get_mouse_position() - get_viewport().get_visible_rect().size * 0.5
 	var to_mouse := mouse - predicted_pos
@@ -142,6 +160,17 @@ func on_welcome(peer_id: int) -> void:
 	GameLog.info("client", "welcome, peer id %d" % peer_id)
 
 
+## Re-number our input stream relative to the server and drop the history that
+## was numbered under the old scheme -- replaying it would apply inputs the
+## server never accepted.
+func _resync_input_tick(server_tick: int, why: String) -> void:
+	input_tick = server_tick + SimConfig.INPUT_TARGET_LEAD
+	pending.clear()
+	_ack_stall = 0
+	_last_acked = -1
+	GameLog.warn("client", "input re-sync: %s" % why)
+
+
 func on_roster(data: PackedByteArray) -> void:
 	roster = NetCodec.decode_roster(data)
 	hud_dirty.emit()
@@ -158,7 +187,9 @@ func on_enter_instance(id: int, kind: int, server_tick: int, boss_id: String,
 	predicted_pos = spawn
 	pending.clear()
 	server_tick_est = server_tick
-	input_tick = server_tick + 8
+	input_tick = server_tick + SimConfig.INPUT_TARGET_LEAD
+	_last_acked = -1
+	_ack_stall = 0
 	my_alive = true
 	my_hp = SimConfig.PLAYER_MAX_HP
 	my_escape = 0.0
@@ -168,6 +199,8 @@ func on_enter_instance(id: int, kind: int, server_tick: int, boss_id: String,
 	# was triggered -- otherwise the synthesised button would keep firing and
 	# bounce the player straight back out of the hub.
 	request_escape = false
+	request_respawn = false
+	my_respawn_wait = 0.0
 	cleared_countdown = Protocol.COUNTDOWN_NONE
 	GameLog.info("client", "entered instance %d (%s)" % [id, Protocol.InstanceKind.keys()[kind]])
 	instance_changed.emit()
@@ -187,10 +220,12 @@ func on_snapshot(data: PackedByteArray) -> void:
 	if server_tick_est < tick or server_tick_est > tick + 12:
 		server_tick_est = tick
 	# Keep the client roughly one buffer ahead of the server so inputs arrive
-	# just before they are needed rather than late.
+	# just before they are needed rather than late. The band is deliberately
+	# narrower than the server's acceptance window (SimConfig.INPUT_MAX_LEAD),
+	# so we always correct before the server starts dropping anything.
 	var lead := input_tick - tick
-	if lead < 2 or lead > 16:
+	if lead < SimConfig.INPUT_LEAD_MIN or lead > SimConfig.INPUT_LEAD_MAX:
-		input_tick = tick + 8
+		_resync_input_tick(tick, "lead %d out of band" % lead)
 
 	for rec: Dictionary in snap["players"]:
 		if int(rec["peer"]) == my_peer:
@@ -206,9 +241,26 @@ func _reconcile(rec: Dictionary) -> void:
 	my_escaping = (int(rec["flags"]) & Protocol.F_ESCAPING) != 0
 	my_spawn_grace = (int(rec["flags"]) & Protocol.F_SPAWN_GRACE) != 0
 	my_escape = float(rec["escape"])
+	my_respawn_wait = float(rec["respawn_wait"])
+	if my_alive:
+		request_respawn = false
 	hud_dirty.emit()
 
 	var acked := int(rec["last_input_tick"])
+
+	# If the server is not consuming anything we send, our numbering is outside
+	# its window; lead alone cannot detect that, because a wrong lead looks
+	# perfectly normal from here. This is what makes the failure self-healing.
+	if acked == _last_acked:
+		_ack_stall += 1
+		if _ack_stall >= SimConfig.INPUT_ACK_STALL_LIMIT:
+			_resync_input_tick(int(snap_curr["tick"]),
+				"server stopped acknowledging input")
+			return
+	else:
+		_last_acked = acked
+		_ack_stall = 0
+
 	while not pending.is_empty() and pending[0].tick <= acked:
 		pending.pop_front()
 

src/net/net_codec.gd

@@ -27,8 +27,6 @@ static func encode_snapshot(world: SimWorld,
 		var flags := 0
 		if p.alive:
 			flags |= Protocol.F_ALIVE
-		if p.iframes > 0:
-			flags |= Protocol.F_INVULN
 		if p.escape_ticks > 0:
 			flags |= Protocol.F_ESCAPING
 		if p.spawn_grace > 0:
@@ -37,6 +35,9 @@ static func encode_snapshot(world: SimWorld,
 			flags |= Protocol.F_LINKDEAD
 		b.put_u8(flags)
 		b.put_u8(clampi(roundi(p.escape_progress() * 255.0), 0, 255))
+		# Whole ticks are more than the HUD needs; a byte of tenths of a second
+		# covers the lockout with room to spare.
+		b.put_u8(clampi(roundi(float(p.respawn_lockout) / 6.0), 0, 255))
 		# Echoed so the owning client knows how far to rewind when reconciling.
 		b.put_u32(p.last_input_tick)
 
@@ -85,6 +86,7 @@ static func decode_snapshot(data: PackedByteArray) -> Dictionary:
 			"hp": b.get_u16(),
 			"flags": b.get_u8(),
 			"escape": float(b.get_u8()) / 255.0,
+			"respawn_wait": float(b.get_u8()) / 10.0,
 			"last_input_tick": b.get_u32(),
 		})
 

src/net/protocol.gd

@@ -4,8 +4,10 @@ extends RefCounted
 ## server refuses mismatched clients at handshake rather than desyncing later.
 
 ## 2: added spawn-grace flag + cleared countdown to the snapshot, and the
-## roster message.
+##    roster message.
-const VERSION := 2
+## 3: dropped the post-hit invulnerability flag, added the respawn-lockout byte
+##    to each player record.
+const VERSION := 3
 const DEFAULT_PORT := 27015
 const MAX_CLIENTS := 32
 
@@ -22,10 +24,9 @@ enum InstanceKind { LOBBY, DUNGEON }
 
 ## Player flags packed into the snapshot's per-player byte.
 const F_ALIVE := 1
-const F_INVULN := 2
 const F_ESCAPING := 4
-## Arrival protection: invulnerable and unable to shoot. Distinct from F_INVULN
+## Arrival protection: invulnerable and unable to shoot. The only invulnerable
-## so the client can label the status rather than just tint the ship.
+## state there is -- see the i-frames note in SimConfig.
 const F_SPAWN_GRACE := 8
 ## The peer behind this player has dropped and is being channelled out.
 const F_LINKDEAD := 16

src/sim/sim_player.gd

@@ -9,10 +9,11 @@ var pos := Vector2.ZERO
 var aim: float = 0.0
 var hp: int = SimConfig.PLAYER_MAX_HP
 var alive: bool = true
-var iframes: int = 0
 var fire_cooldown: int = 0
 ## Ticks of arrival protection left: invulnerable, and unable to shoot.
 var spawn_grace: int = 0
+## Ticks before a downed player may ask to return to the hub.
+var respawn_lockout: int = 0
 
 ## The peer's connection dropped, but the player is deliberately still in the
 ## world. Held here rather than deleted so a disconnect cannot be used to dodge
@@ -37,9 +38,10 @@ func escape_progress() -> float:
 	return clampf(float(escape_ticks) / float(SimConfig.ESCAPE_CHANNEL_TICKS), 0.0, 1.0)
 
 
-## Invulnerable while arriving, or in the usual post-hit window.
+## Arrival protection only -- there is no post-hit invulnerability. See the
+## note on SimConfig.SPAWN_GRACE_TICKS.
 func invulnerable() -> bool:
-	return spawn_grace > 0 or iframes > 0
+	return spawn_grace > 0
 
 
 ## Arrival protection also locks the gun, so the spawn point is not a free
@@ -54,8 +56,13 @@ func reset_for_instance(spawn: Vector2, grace: int = 0) -> void:
 	pos = spawn
 	hp = SimConfig.PLAYER_MAX_HP
 	alive = true
-	iframes = 0
 	spawn_grace = grace
+	respawn_lockout = 0
 	fire_cooldown = 0
 	escape_ticks = 0
 	input_queue.clear()
+
+
+## True once the lockout has run out and the hub is available again.
+func can_request_respawn() -> bool:
+	return not alive and respawn_lockout <= 0

src/sim/sim_world.gd

@@ -149,8 +149,6 @@ func step() -> void:
 
 func _step_players() -> void:
 	for p in players.values():
-		if p.iframes > 0:
-			p.iframes -= 1
 		if p.spawn_grace > 0:
 			p.spawn_grace -= 1
 		if p.fire_cooldown > 0:
@@ -159,10 +157,13 @@ func _step_players() -> void:
 		var frame := _take_input(p)
 
 		if not p.alive:
+			if p.respawn_lockout > 0:
+				p.respawn_lockout -= 1
 			# No timed respawn: a downed player waits for the hub. Asking to go
 			# is an input like any other, so a dead client cannot be revived by
-			# anything except its own request reaching the server.
+			# anything except its own request reaching the server -- and not
-			if frame.pressed(InputFrame.BTN_INTERACT) or p.linkdead:
+			# before the lockout expires, however early its UI lets it ask.
+			if p.can_request_respawn() and (frame.pressed(InputFrame.BTN_INTERACT) or p.linkdead):
 				events.append({"t": SimEvent.Type.RESPAWN_REQUESTED, "peer": p.peer_id})
 			continue
 
@@ -372,11 +373,11 @@ func _kill_bullet(slot: int) -> void:
 ## SimConfig.ESCAPE_CHANNEL_TICKS for why that would reward pulling the plug.
 func _damage_player(p: SimPlayer, amount: int) -> void:
 	p.hp = maxi(p.hp - amount, 0)
-	p.iframes = SimConfig.PLAYER_IFRAMES
 	events.append({"t": SimEvent.Type.PLAYER_HIT, "peer": p.peer_id, "dmg": amount, "hp": p.hp})
 	if p.hp <= 0:
 		p.alive = false
 		p.escape_ticks = 0
+		p.respawn_lockout = SimConfig.RESPAWN_LOCKOUT_TICKS
 		events.append({"t": SimEvent.Type.PLAYER_DIED, "peer": p.peer_id})
 
 

src/ui/hud.gd

@@ -6,9 +6,12 @@ const MARGIN := 24.0
 const BAR_W := 260.0
 const BAR_H := 16.0
 
+signal respawn_pressed
+
 var _canvas: Control
 var _status: Label
 var _hint: Label
+var _respawn_button: Button
 var _hit_flash: float = 0.0
 var client: ClientRuntime = null
 
@@ -38,6 +41,20 @@ func _ready() -> void:
 	_hint.offset_left = MARGIN
 	_hint.offset_top = -56.0
 
+	# A real Button rather than something drawn in _draw_hud: this is the one
+	# HUD element the player has to actually click, and it starts disabled so
+	# death is not exited by whatever key happened to be under a finger.
+	_respawn_button = Button.new()
+	_respawn_button.custom_minimum_size = Vector2(240.0, 40.0)
+	_respawn_button.set_anchors_and_offsets_preset(Control.PRESET_CENTER)
+	_respawn_button.offset_left = -120.0
+	_respawn_button.offset_right = 120.0
+	_respawn_button.offset_top = 40.0
+	_respawn_button.offset_bottom = 80.0
+	_respawn_button.pressed.connect(func() -> void: respawn_pressed.emit())
+	_respawn_button.visible = false
+	_canvas.add_child(_respawn_button)
+
 
 func _make_label(pos: Vector2) -> Label:
 	var l := Label.new()
@@ -53,9 +70,23 @@ func _process(delta: float) -> void:
 	_hit_flash = maxf(_hit_flash - delta * 2.5, 0.0)
 	_status.text = _status_text()
 	_hint.text = _hint_text()
+	_update_respawn_button()
 	_canvas.queue_redraw()
 
 
+func _update_respawn_button() -> void:
+	var downed := client != null and not client.my_alive
+	_respawn_button.visible = downed
+	if not downed:
+		return
+	# Mirrors SimConfig.RESPAWN_LOCKOUT_TICKS, which the server enforces
+	# independently -- this is presentation, not the rule.
+	var ready := client.my_respawn_wait <= 0.0
+	_respawn_button.disabled = not ready
+	_respawn_button.text = "Return to hub" if ready \
+		else "Return to hub  (%.0fs)" % ceil(client.my_respawn_wait)
+
+
 func _status_text() -> String:
 	if client == null:
 		return "connecting..."
@@ -69,7 +100,7 @@ func _hint_text() -> String:
 	if client == null:
 		return ""
 	if not client.my_alive:
-		return "DOWN   --   press E to return to the hub"
+		return "DOWN"
 	if client.instance_kind == Protocol.InstanceKind.LOBBY:
 		return "WASD move   mouse aim   LMB fire   E on the ring to enter a dungeon   Esc menu"
 	return "WASD move   mouse aim   LMB fire   hold F to return to the hub   Esc menu"
@@ -106,9 +137,7 @@ func _draw_hud() -> void:
 		var centre := _canvas.size * 0.5
 		_canvas.draw_string(ThemeDB.fallback_font, centre - Vector2(52.0, 8.0),
 			"DOWN", HORIZONTAL_ALIGNMENT_LEFT, -1, 34, Color(1.0, 0.4, 0.4))
-		_canvas.draw_string(ThemeDB.fallback_font, centre - Vector2(118.0, -18.0),
+
-			"press E to return to the hub", HORIZONTAL_ALIGNMENT_LEFT, -1, 16,
-			Color(1.0, 0.75, 0.75))
 
 	if _hit_flash > 0.0:
 		_canvas.draw_rect(Rect2(Vector2.ZERO, _canvas.size),

src/ui/main_menu.gd

@@ -9,6 +9,8 @@ var _address: LineEdit
 var _port: LineEdit
 var _name: LineEdit
 var _status: Label
+var _join_button: Button
+var _host_button: Button
 
 
 func _ready() -> void:
@@ -43,15 +45,15 @@ func _ready() -> void:
 	_address = _field(panel, "server address", GameOpts.host_address)
 	_port = _field(panel, "port", str(GameOpts.port))
 
-	var join := Button.new()
+	_join_button = Button.new()
-	join.text = "Join"
+	_join_button.text = "Join"
-	join.pressed.connect(_on_join)
+	_join_button.pressed.connect(_on_join)
-	panel.add_child(join)
+	panel.add_child(_join_button)
 
-	var host := Button.new()
+	_host_button = Button.new()
-	host.text = "Host and play (listen server)"
+	_host_button.text = "Host and play (listen server)"
-	host.pressed.connect(_on_host)
+	_host_button.pressed.connect(_on_host)
-	panel.add_child(host)
+	panel.add_child(_host_button)
 
 	var quit := Button.new()
 	quit.text = "Quit"
@@ -78,6 +80,13 @@ func set_status(text: String) -> void:
 	_status.text = text
 
 
+## Locks the connect buttons while an attempt is in flight, so a second click
+## cannot start a competing connection over the top of the first.
+func set_busy(busy: bool) -> void:
+	_join_button.disabled = busy
+	_host_button.disabled = busy
+
+
 func _commit_name() -> void:
 	var clean := _name.text.strip_edges()
 	if not clean.is_empty():

src/view/game_scene.gd

@@ -15,6 +15,7 @@ func _ready() -> void:
 	get_viewport().size_changed.connect(_recentre)
 	menu.return_to_hub_requested.connect(_on_return_to_hub)
 	menu.disconnect_requested.connect(_on_disconnect)
+	hud.respawn_pressed.connect(_on_respawn_pressed)
 
 
 func _recentre() -> void:
@@ -42,5 +43,11 @@ func _on_disconnect() -> void:
 	Net.shutdown()
 
 
+## Sets the intent bit; the server decides whether the lockout has expired.
+func _on_respawn_pressed() -> void:
+	if _bound != null:
+		_bound.request_respawn = true
+
+
 func _on_local_hit(_damage: int) -> void:
 	hud.flash_hit()

tests/integration/test_replica_parity.gd

@@ -81,7 +81,6 @@ func test_bullets_killed_by_a_hit_are_removed_from_the_replica() -> void:
 	server.add_player(1, "tester")
 	# Just below the boss, inside the ring pattern rather than at its centre.
 	server.players[1].pos = Vector2(0.0, 20.0)
-	server.players[1].iframes = 0
 	server.spawn_boss(Content.warden())
 	var hits := 0
 	for _i in 900:

tests/unit/test_emergency_escape.gd

@@ -9,7 +9,6 @@ const PEER := 3
 func before_each() -> void:
 	world = SimWorld.new(1)
 	world.add_player(PEER, "tester")
-	world.players[PEER].iframes = 0
 
 
 func _hold_escape(ticks: int) -> void:
@@ -65,7 +64,6 @@ func test_dying_ends_the_channel() -> void:
 	_hold_escape(20)
 	var p: SimPlayer = world.players[PEER]
 	p.hp = 1
-	p.iframes = 0
 	world.pool.spawn(p.pos, Vector2.ZERO, 6.0, 60, 999,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
 	world.step()
@@ -91,7 +89,6 @@ func test_a_linkdead_player_is_still_killable_while_channelling() -> void:
 	var p: SimPlayer = world.players[PEER]
 	p.linkdead = true
 	p.hp = 1
-	p.iframes = 0
 	world.step()
 	world.pool.spawn(p.pos, Vector2.ZERO, 6.0, 60, 999,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)

tests/unit/test_input_lead.gd

@@ -0,0 +1,70 @@
+extends GutTest
+## The client numbers its input frames a fixed "lead" ahead of the server's
+## tick, and the server accepts a window around its own tick. These two numbers
+## have to be chosen together.
+##
+## They were not, and the result was the worst kind of bug: the server silently
+## dropped every input from a client whose lead had drifted into the gap between
+## "server stops accepting" and "client notices", the client never corrected
+## because by its own reckoning nothing was wrong, and the server coasted on a
+## stale input forever. It needed two independent clocks to drift apart, so it
+## only appeared after a few minutes -- and then never went away.
+
+var world: SimWorld
+const PEER := 5
+
+
+func before_each() -> void:
+	world = SimWorld.new(1)
+	world.add_player(PEER, "tester")
+	world.tick = 1000
+
+
+func _queue_at_lead(lead: int) -> void:
+	var frames: Array[InputFrame] = [
+		InputFrame.make(world.tick + lead, Vector2.RIGHT, 0.0, 0)]
+	world.queue_input(PEER, frames)
+
+
+## The invariant that was violated. The client corrects itself at
+## INPUT_LEAD_MAX; the server must keep accepting well past that, or there is a
+## band where the server refuses input the client still thinks is fine.
+func test_the_server_accepts_every_lead_the_client_tolerates() -> void:
+	assert_gt(SimConfig.INPUT_MAX_LEAD, SimConfig.INPUT_LEAD_MAX,
+		"the server's acceptance window must be wider than the client's " +
+		"correction band, or drift lands in a silent dead zone")
+
+
+func test_inputs_across_the_whole_client_band_are_accepted() -> void:
+	for lead in range(SimConfig.INPUT_LEAD_MIN, SimConfig.INPUT_LEAD_MAX + 1):
+		world.players[PEER].input_queue.clear()
+		world.players[PEER].last_input_tick = 0
+		_queue_at_lead(lead)
+		assert_eq(world.players[PEER].input_queue.size(), 1,
+			"lead %d is inside the band the client will not correct, so the " % lead +
+			"server has to accept it")
+
+
+func test_absurd_lead_is_still_rejected() -> void:
+	_queue_at_lead(SimConfig.INPUT_MAX_LEAD + 1)
+	assert_eq(world.players[PEER].input_queue.size(), 0,
+		"the window is wider, not gone -- a client claiming a far-future tick " +
+		"still buys nothing")
+
+
+## What the drift actually looked like from the server's side: nothing arrives,
+## so it coasts on the last input it saw. That is correct behaviour for a brief
+## hiccup and catastrophic as a permanent state, which is why the client now
+## has a stall detector rather than relying on its lead estimate alone.
+func test_a_starved_server_coasts_then_stops() -> void:
+	var p: SimPlayer = world.players[PEER]
+	_queue_at_lead(SimConfig.INPUT_TARGET_LEAD)
+	world.step()
+	var moved_once: Vector2 = p.pos
+	assert_ne(moved_once, Vector2.ZERO, "setup: the input should have moved us")
+	for _i in SimConfig.INPUT_MAX_AGE + 5:
+		world.step()
+	var coasted: Vector2 = p.pos
+	for _i in 60:
+		world.step()
+	assert_eq(p.pos, coasted, "coasting has to end, or a silent client drifts forever")

tests/unit/test_input_lead.gd.uid

@@ -0,0 +1 @@
+uid://xemxybh1gvvo

tests/unit/test_server_authority.gd

@@ -91,7 +91,6 @@ func test_a_starved_player_eventually_stops_moving() -> void:
 func test_enemy_bullets_damage_the_player_and_are_consumed() -> void:
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
-	p.iframes = 0
 	world.pool.spawn(Vector2(-1.0, 0.0), Vector2.ZERO, 6.0, 60, 25,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
 	world.step()
@@ -99,22 +98,37 @@ func test_enemy_bullets_damage_the_player_and_are_consumed() -> void:
 	assert_eq(world.pool.live_count, 0, "a bullet that hits must be consumed")
 
 
-func test_invulnerability_frames_stop_a_second_hit() -> void:
+## The inverse of what this asserted before. There are no i-frames: in a bullet
+## hell the wall is the threat, and letting one hit grant immunity to the rest
+## of the wall inverts the whole risk model.
+func test_every_bullet_in_a_wall_lands() -> void:
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
-	p.iframes = 0
+	for _i in 3:
-	for _i in 2:
 		world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
 			SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
 	world.step()
-	assert_eq(p.hp, SimConfig.PLAYER_MAX_HP - 25, "two bullets in one tick is still one hit")
+	assert_eq(p.hp, SimConfig.PLAYER_MAX_HP - 75,
+		"three bullets is three hits, not one")
+
+
+func test_consecutive_ticks_both_deal_damage() -> void:
+	var p: SimPlayer = world.players[PEER]
+	p.pos = Vector2.ZERO
+	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 10,
+		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
+	world.step()
+	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 10,
+		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
+	world.step()
+	assert_eq(p.hp, SimConfig.PLAYER_MAX_HP - 20,
+		"there is no window of immunity after a hit")
 
 
 func test_a_replica_world_never_resolves_a_hit() -> void:
 	world.authoritative = false
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
-	p.iframes = 0
 	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
 	world.step()
@@ -126,7 +140,6 @@ func _kill_player() -> SimPlayer:
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
 	p.hp = 5
-	p.iframes = 0
 	p.spawn_grace = 0
 	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)
@@ -150,6 +163,7 @@ func test_a_downed_player_stays_down_without_input() -> void:
 
 func test_a_downed_player_asking_to_respawn_is_reported_once_per_tick() -> void:
 	_kill_player()
+	_drive(SimConfig.RESPAWN_LOCKOUT_TICKS)
 	world.drain_events()
 	_drive(1, Vector2.ZERO, InputFrame.BTN_INTERACT)
 	assert_eq(_events_of(SimEvent.Type.RESPAWN_REQUESTED).size(), 1,
@@ -159,6 +173,28 @@ func test_a_downed_player_asking_to_respawn_is_reported_once_per_tick() -> void:
 	assert_false(world.players[PEER].alive)
 
 
+## The HUD disables its button for the same duration, but that is presentation.
+## A client that ignores its own UI and spams the bit still has to wait.
+func test_the_respawn_lockout_is_enforced_by_the_server() -> void:
+	_kill_player()
+	world.drain_events()
+	_drive(SimConfig.RESPAWN_LOCKOUT_TICKS - 2, Vector2.ZERO, InputFrame.BTN_INTERACT)
+	assert_eq(_events_of(SimEvent.Type.RESPAWN_REQUESTED).size(), 0,
+		"holding the request down must not shorten the lockout")
+	_drive(4, Vector2.ZERO, InputFrame.BTN_INTERACT)
+	assert_gt(_events_of(SimEvent.Type.RESPAWN_REQUESTED).size(), 0,
+		"and it must actually open once the lockout expires")
+
+
+func test_the_lockout_is_reported_to_the_client() -> void:
+	_kill_player()
+	var snap := NetCodec.decode_snapshot(NetCodec.encode_snapshot(world))
+	var wait: float = snap["players"][0]["respawn_wait"]
+	assert_almost_eq(wait,
+		float(SimConfig.RESPAWN_LOCKOUT_TICKS) / float(SimConfig.TICK_RATE), 0.3,
+		"the HUD needs the countdown to render its disabled button")
+
+
 func test_respawn_request_never_reaches_the_client() -> void:
 	_kill_player()
 	_drive(1, Vector2.ZERO, InputFrame.BTN_INTERACT)
@@ -171,7 +207,6 @@ func test_respawn_request_never_reaches_the_client() -> void:
 func test_spawn_grace_blocks_damage_and_firing() -> void:
 	var p: SimPlayer = world.players[PEER]
 	p.pos = Vector2.ZERO
-	p.iframes = 0
 	p.spawn_grace = 60
 	world.pool.spawn(Vector2.ZERO, Vector2.ZERO, 6.0, 60, 25,
 		SimConfig.TEAM_ENEMY, SimConfig.KIND_ORB)

tools/diag_prediction.gd

@@ -8,10 +8,19 @@ extends Node
 ## Runs the real listen-server path (ServerRuntime + ClientRuntime + the actual
 ## Net loopback), not a mock, so node process order is exactly production's.
 
+## Tick at which to shove the client's input numbering out of sync, simulating
+## the clock drift between two independent processes that produced the original
+## "ship and bullets separate after a few minutes, permanently" bug. 0 disables.
+const DRIFT_AT := 120
+const DRIFT_BY := 14
+
 var _ticks: int = 0
 var _worst: float = 0.0
 var _sum: float = 0.0
 var _samples: int = 0
+## Measured only after the injected drift, which is the interesting window.
+var _worst_after_drift: float = 0.0
+var _drifted: bool = false
 
 
 func _ready() -> void:
@@ -42,6 +51,17 @@ func _physics_process(_delta: float) -> void:
 		_sum += delta_px
 		_samples += 1
 
+	if DRIFT_AT > 0 and _ticks == DRIFT_AT:
+		# Exactly what two drifting clocks do, only instantly. With the old
+		# constants this landed in the gap between "server stops accepting" and
+		# "client notices", and the two never re-converged.
+		cli.input_tick += DRIFT_BY
+		_drifted = true
+		print("--- injected +%d tick input drift ---" % DRIFT_BY)
+	# Give the client a moment to notice before judging it.
+	if _drifted and _ticks > DRIFT_AT + 90:
+		_worst_after_drift = maxf(_worst_after_drift, delta_px)
+
 	if _ticks % 20 == 0:
 		print("tick=%4d srv_tick=%4d srv_pos=%-22s pred=%-22s gap=%6.2fpx acked=%4d cli_tick=%4d queue=%d" % [
 			_ticks, inst.world.tick, str(sp.pos.round()), str(cli.predicted_pos.round()),
@@ -55,5 +75,12 @@ func _physics_process(_delta: float) -> void:
 		print("muzzle offset = %.1fpx, drawn ship radius = %.1fpx, hitbox = %.1fpx" % [
 			SimConfig.PLAYER_MUZZLE_OFFSET, SimConfig.PLAYER_VISUAL_RADIUS,
 			SimConfig.PLAYER_RADIUS])
+		var ok := true
+		if _drifted:
+			print("RECOVERED gap after drift: worst=%.2fpx" % _worst_after_drift)
+			# One tick of movement is 4px; anything near that means the client
+			# re-synced. Tens of px means it is stuck, which is the bug.
+			ok = _worst_after_drift < 12.0
+			print("DRIFT_RECOVERY_" + ("PASS" if ok else "FAIL"))
 		Net.shutdown()
-		get_tree().quit(0)
+		get_tree().quit(0 if ok else 1)