Keep track of channel join status

This makes us behave better when we receive a self-PART message
from the server.

components/app.js

@@ -1023,7 +1023,9 @@ export default class App extends Component {
 			}
 			break;
 		case BufferType.CHANNEL:
-			client.send({ command: "PART", params: [buf.name] });
+			if (buf.joined) {
+				client.send({ command: "PART", params: [buf.name] });
+			}
 			// fallthrough
 		case BufferType.NICK:
 			this.switchBuffer({ name: SERVER_BUFFER });
@@ -1150,8 +1152,16 @@ export default class App extends Component {
 		});
 	}
 
-	handleJoinClick(serverID) {
-		this.openDialog("join", { server: serverID });
+	handleJoinClick(buf) {
+		switch (buf.type) {
+		case BufferType.SERVER:
+			this.openDialog("join", { server: buf.server });
+			break;
+		case BufferType.CHANNEL:
+			let client = this.clients.get(buf.server);
+			client.send({ command: "JOIN", params: [buf.name] });
+			break;
+		}
 	}
 
 	handleJoinSubmit(data) {
@@ -1351,7 +1361,7 @@ export default class App extends Component {
 						bouncerNetwork=${activeBouncerNetwork}
 						onChannelClick=${this.handleChannelClick}
 						onClose=${() => this.close(activeBuffer)}
-						onJoin=${() => this.handleJoinClick(activeBuffer.server)}
+						onJoin=${() => this.handleJoinClick(activeBuffer)}
 						onAddNetwork=${this.handleAddNetworkClick}
 						onManageNetwork=${() => this.handleManageNetworkClick(activeBuffer.server)}
 					/>

components/buffer-header.js

@@ -117,13 +117,27 @@ export default function BufferHeader(props) {
 		if (props.buffer.topic) {
 			description = linkify(stripANSI(props.buffer.topic), props.onChannelClick);
 		}
-		actions = html`
-			<button
-				key="part"
-				class="danger"
-				onClick=${handleCloseClick}
-			>Leave</button>
-		`;
+		if (props.buffer.joined) {
+			actions = html`
+				<button
+					key="part"
+					class="danger"
+					onClick=${handleCloseClick}
+				>Leave</button>
+			`;
+		} else {
+			actions = html`
+				<button
+					key="join"
+					onClick=${handleJoinClick}
+				>Join</button>
+				<button
+					key="part"
+					class="danger"
+					onClick=${handleCloseClick}
+				>Close</button>
+			`;
+		}
 		break;
 	case BufferType.NICK:
 		if (props.user) {