Treeview Jitter

# May need high keyboard repeat rate to be observable.
# xset r rate 100 100; wish tvjitter.tcl; xset r rate

bind . <Control-q> exit

proc randstr {{length {6}}} {
	set str ""
	for {set i 0} {$i < $length} {incr i} {
		append str [format %c [expr {97 + int(rand() * 26)}]]
	}
	return $str
}

set extra [list]
#lappend extra -show headings

grid [ttk::treeview .t -columns {a b c} -height 30 {*}$extra]
bind .t <Key-Down> [list down]

proc down {} {
	set item [.t next [.t focus]]
	if {$item eq ""} { return -code break }
	set index [.t index $item]
	.t focus $item

	# Try with one of these. Is there noticeable jitter?
	.t see $item
	#if {$index > 29} {.t yview scroll 1 units}
	#.t yview moveto [expr {double($index - 29) / 500.0}]

	.t selection set [list $item]
	return -code break
}

for {set i 0} {$i < 500} {incr i} {
	.t insert {} end -values [list $i [randstr] [randstr]]
}

focus .t
.t focus I001
.t selection set I001