Strange behaviour of text widget

I have written the following code which reproducibly causes a strange
result. The text string used as input is actually 176 characters long
(according to the 'string length' command). Just in case it matters, this
text string contains with no internal line breaks.

text .txt -width 55 -wrap word
label .lbl -bg cyan3

pack .lbl -side top -fill x
pack .txt -side left

set comment "Science fiction author, journalist, and all around futurist
Cory Doctorow believes the recent copyright battles are the opening salvo of
a larger war against general computers."

..txt delete 1.0 end
..txt insert 1.0 $comment

update idle

set txtLines [.txt count -displaylines 1.0 end]
puts $txtLines

.....
.....

When the 'update idle' line of code is *not* used, the output from the puts
statement is 176. When the update line IS used, the result is 4.

Given that I was trying to get the count of the display lines produced,
these outcomes are confusing. From the visual evidence (text box), 4 is
obviously the correct outcome.

I have no idea what is causing this strange behaviour so am posting it here
so someone with a detailed understanding of the internals behind the text
widget can see it and offer an explanation and indication of whether I
should post a bug report.

TIA

Rob.
.

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