xP: avoid expensive updates/refreshes

xP/public/xP.js

@@ -286,7 +286,6 @@ rpcEventHandlers.set(Relay.Event.BufferLine, e => {
 	// Initial sync: skip all other processing, let highlights be.
 	if (bufferCurrent === undefined) {
 		b.lines.push(line)
-		bufferPopExcessLines(b)
 		return
 	}
 
@@ -302,7 +301,13 @@ rpcEventHandlers.set(Relay.Event.BufferLine, e => {
 		else
 			b.newMessages++
 	}
-	bufferPopExcessLines(b)
+
+	// XXX: In its unkeyed diff algorithm, Mithril.js can only efficiently
+	// deal with common prefixes, i.e., indefinitely growing buffers.
+	// But we don't want to key all children of Buffer,
+	// so only trim buffers while they are, or once they become invisible.
+	if (e.bufferName != bufferCurrent)
+		bufferPopExcessLines(b)
 
 	if (e.leakToActive) {
 		let bc = buffers.get(bufferCurrent)
@@ -313,7 +318,6 @@ rpcEventHandlers.set(Relay.Event.BufferLine, e => {
 			else
 				bc.newMessages++
 		}
-		bufferPopExcessLines(bc)
 	}
 
 	if (line.isHighlight || (!visible && !line.isUnimportant &&
@@ -370,8 +374,16 @@ rpcEventHandlers.set(Relay.Event.BufferRemove, e => {
 
 rpcEventHandlers.set(Relay.Event.BufferActivate, e => {
 	let old = buffers.get(bufferCurrent)
-	if (old !== undefined)
+	if (old !== undefined) {
 		bufferResetStats(old)
+		bufferPopExcessLines(old)
+	}
+
+	// Initial sync: trim all buffers to our limit, just for consistency.
+	if (bufferCurrent === undefined) {
+		for (let b of buffers.values())
+			bufferPopExcessLines(b)
+	}
 
 	bufferLast = bufferCurrent
 	let b = buffers.get(e.bufferName)