Something recently has stopped this from working. On investigation this seems to be due to mobile.styles and specifically something in the stylesheets/common/mf-typography.css file.

Bizarrely this seems to be due to this rule:
.content {

margin: 14pt 23px 0 23px;

}

I notice when I change this to:
.content {

margin: 14pt 17px 0 23px;

}
or
.content {

margin: 14pt 23px 0 17px;

}
it works

It seems that if the inner horizontal width of the main containing element is less 280px it will not show the reader function

e.g.
width: 279px; it will show
width: 280px; no reader function

40px appears to be a magic number

Version: unspecified
Severity: enhancement