Add an option to spectate from battle start

play.pokemonshowdown.com/src/client-main.ts

@@ -120,6 +120,7 @@ class PSPrefs extends PSStreamModel<string | null> {
 	ignoreopp: boolean | null = null;
 	autotimer: boolean | null = null;
 	autohardcore: boolean | null = null;
+	spectatefromstart: boolean | null = null;
 	rightpanelbattles: boolean | null = null;
 	disallowspectators: boolean | null = null;
 	starredformats: { [formatid: string]: true | undefined } | null = null;

play.pokemonshowdown.com/src/panel-battle.tsx

@@ -147,6 +147,8 @@ export class BattleRoom extends ChatRoom {
 	request: BattleRequest | null = null;
 	choices: BattleChoiceBuilder | null = null;
 	autoTimerActivated: boolean | null = null;
+	/** null = initializing */
+	rejoining: boolean | null = null;
 
 	loadReplay() {
 		const replayid = this.id.slice(7);
@@ -360,7 +362,7 @@ class BattlePanel extends PSRoomPanel<BattleRoom> {
 		scene.tooltips.listen($elem.find('.battle-controls-container'));
 		scene.tooltips.listen(scene.log.elem);
 		super.componentDidMount();
-		battle.seekTurn(Infinity);
+		this.fastForwardIfRejoining();
 		if (PS.prefs.autohardcore) {
 			battle.setHardcoreMode(true);
 		}
@@ -380,11 +382,18 @@ class BattlePanel extends PSRoomPanel<BattleRoom> {
 			this.battleHeight = 360;
 		}
 	}
+	fastForwardIfRejoining() {
+		const room = this.props.room;
+		if (!room.rejoining || !room.side) return;
+		room.rejoining = false;
+		if (!PS.prefs.spectatefromstart) room.battle.seekTurn(Infinity);
+	}
 	override receiveLine(args: Args) {
 		const room = this.props.room;
 		switch (args[0]) {
 		case 'initdone':
-			room.battle.seekTurn(Infinity);
+			room.rejoining ||= false;
+			this.fastForwardIfRejoining();
 			return;
 		case 'request':
 			this.receiveRequest(args[1] ? JSON.parse(args[1]) : null);
@@ -395,6 +404,9 @@ class BattlePanel extends PSRoomPanel<BattleRoom> {
 		case 'c': case 'c:': case 'chat': case 'chatmsg': case 'inactive':
 			room.battle.instantAdd('|' + args.join('|'));
 			return;
+		case 'start':
+			room.rejoining ??= true;
+			break;
 		case 'error':
 			if (args[1].startsWith('[Invalid choice]') && room.request) {
 				room.choices = new BattleChoiceBuilder(room.request);
@@ -421,9 +433,11 @@ class BattlePanel extends PSRoomPanel<BattleRoom> {
 		BattleChoiceBuilder.fixRequest(request, room.battle);
 
 		if (request.side) {
+			const wasPlayer = !!room.side;
 			room.battle.myPokemon = request.side.pokemon;
 			room.battle.setViewpoint(request.side.id);
 			room.side = request.side;
+			if (!wasPlayer) this.fastForwardIfRejoining();
 		}
 		if (request.ally) {
 			room.battle.myAllyPokemon = request.ally.pokemon;

play.pokemonshowdown.com/src/panel-popups.tsx

@@ -1571,6 +1571,10 @@ class BattleOptionsPanel extends PSRoomPanel {
 			}
 			break;
 		}
+		case 'spectatefromstart': {
+			PS.prefs.set('spectatefromstart', value);
+			break;
+		}
 		case 'ignoreopp': {
 			PS.prefs.set('ignoreopp', value);
 			this.handleIgnoreOpponent(value);
@@ -1692,6 +1696,14 @@ class BattleOptionsPanel extends PSRoomPanel {
 					/> Automatically enable hardcore mode
 				</label>
 			</p>
+			<p>
+				<label class="checkbox">
+					<input
+						name="spectatefromstart" checked={!!PS.prefs.spectatefromstart}
+						type="checkbox" onChange={this.handleAllSettings}
+					/> Start at turn 0 when spectating battles
+				</label>
+			</p>
 			{!PS.prefs.onepanel && document.body.offsetWidth >= 800 && <p>
 				<label class="checkbox">
 					<input