xP: beep on highlight

800 Hz seems like it could match a POST beep.

xP/public/xP.js

@@ -127,6 +127,32 @@ class RelayRpc extends EventTarget {
 	}
 }
 
+// ---- Utilities --------------------------------------------------------------
+
+// On macOS, the Alt/Option key transforms characters, which basically breaks
+// all event.altKey shortcuts, so require combining them with Control as well
+// on that system.
+function hasShortcutModifiers(event) {
+	// This method of detection only works with Blink browsers, as of writing.
+	return event.altKey && !event.metaKey &&
+		(navigator.userAgentData?.platform === 'macOS') === event.ctrlKey
+}
+
+const audioContext = new AudioContext()
+
+function beep() {
+	let gain = audioContext.createGain()
+	gain.gain.value = 0.5
+	gain.connect(audioContext.destination)
+
+	let oscillator = audioContext.createOscillator()
+	oscillator.type = "triangle"
+	oscillator.frequency.value = 800
+	oscillator.connect(gain)
+	oscillator.start(audioContext.currentTime)
+	oscillator.stop(audioContext.currentTime + 0.1)
+}
+
 // ---- Event processing -------------------------------------------------------
 
 let rpc = new RelayRpc(proxy)
@@ -300,8 +326,11 @@ rpc.addEventListener('BufferLine', event => {
 
 	// TODO: Find some way of highlighting the tab in a browser.
 	// TODO: Also highlight on unseen private messages, like xC does.
-	if (!visible && line.isHighlight)
+	if (line.isHighlight) {
-		b.highlighted = true
+		beep()
+		if (!visible)
+			b.highlighted = true
+	}
 })
 
 rpc.addEventListener('BufferClear', event => {
@@ -331,15 +360,6 @@ for (let i = 0; i < 24; i++) {
 
 // ---- UI ---------------------------------------------------------------------
 
-// On macOS, the Alt/Option key transforms characters, which basically breaks
-// all event.altKey shortcuts, so require combining them with Control as well
-// on that system.
-function hasShortcutModifiers(event) {
-	// This method of detection only works with Blink browsers, as of writing.
-	return event.altKey && !event.metaKey &&
-		(navigator.userAgentData?.platform === 'macOS') === event.ctrlKey
-}
-
 let linkRE = [
 	/https?:\/\//,
 	/([^\[\](){}<>"'\s]|\([^\[\](){}<>"'\s]*\))+/,
@@ -616,11 +636,12 @@ let Input = {
 		if (b === undefined)
 			return false
 
-		// TODO: Ding otherwise.
 		if (b.historyAt > 0) {
 			if (b.historyAt == b.history.length)
 				b.input = textarea.value
 			textarea.value = b.history[--b.historyAt]
+		} else {
+			beep()
 		}
 		return true
 	},
@@ -630,12 +651,13 @@ let Input = {
 		if (b === undefined)
 			return false
 
-		// TODO: Ding otherwise.
 		if (b.historyAt < b.history.length) {
 			if (++b.historyAt == b.history.length)
 				textarea.value = b.input
 			else
 				textarea.value = b.history[b.historyAt]
+		} else {
+			beep()
 		}
 		return true
 	},