The only way I know of is to make sure the text on which you put the
display property is never too close to the right edge of the window.
The bug was that the display engine considered the X coordinate of the
buffer text in the text area when it made the decision whether to
decrease the width of the string displayed in the margin.
(Btw I am off-list, if you can reply-all I would much appreciate it!)
I will try to remember, but I cannot promise, as I always reply only
to the list in this case, without thinking. (I almost blew it this
time.)