diff --git a/tools/vt/linebuf.go b/tools/vt/linebuf.go
index 121b090d0..73d02f1b9 100644
--- a/tools/vt/linebuf.go
+++ b/tools/vt/linebuf.go
@@ -6,21 +6,26 @@ import (
 
 var _ = fmt.Print
 
+const extra_capacity uint = 4
+
 type LineBuf struct {
-	cells      []Cell
-	attrs      []LineAttrs
-	line_map   []uint
-	xnum, ynum uint
+	cells             []Cell
+	attrs             []LineAttrs
+	line_map, scratch []uint
+	xnum, ynum        uint
 }
 
 func NewLineBuf(xnum, ynum uint) *LineBuf {
-	lm := make([]uint, ynum, ynum)
+	lm := make([]uint, ynum, ynum+extra_capacity)
 	var i uint
 	for i = 0; i < ynum; i++ {
 		lm[i] = i
 	}
 	return &LineBuf{
-		cells: make([]Cell, xnum*ynum, xnum*ynum), attrs: make([]LineAttrs, ynum, ynum), xnum: xnum, ynum: ynum, line_map: lm,
+		cells:   make([]Cell, xnum*ynum, xnum*(ynum+extra_capacity)),
+		attrs:   make([]LineAttrs, len(lm), cap(lm)),
+		scratch: make([]uint, len(lm), cap(lm)),
+		xnum:    xnum, ynum: ynum, line_map: lm,
 	}
 }
 
@@ -32,11 +37,11 @@ func (self *LineBuf) Line(y uint) Line {
 func (self *LineBuf) AddLines(n uint) {
 	ynum := self.ynum + n
 	if uint(cap(self.line_map)) >= ynum {
-		self.line_map = self.line_map[:ynum]
-		self.attrs = self.attrs[:ynum]
-		self.cells = self.cells[:ynum*self.xnum]
+		self.line_map = self.line_map[0:ynum:cap(self.line_map)]
+		self.scratch = self.scratch[0:ynum:cap(self.scratch)]
+		self.attrs = self.attrs[0:ynum:cap(self.attrs)]
+		self.cells = self.cells[0 : ynum*self.xnum : cap(self.cells)]
 	} else {
-		const extra_capacity uint = 2
 		newattrs := make([]LineAttrs, ynum, ynum+extra_capacity)
 		copy(newattrs, self.attrs)
 		newlinemap := make([]uint, ynum, ynum+extra_capacity)
@@ -49,6 +54,7 @@ func (self *LineBuf) AddLines(n uint) {
 		self.attrs = newattrs
 		self.line_map = newlinemap
 		self.cells = newcells
+		self.scratch = make([]uint, len(self.line_map), cap(self.line_map))
 	}
 	self.ynum = ynum
 }