Thanks for the reply !
The progress bar tag (%pb) is before the viewports and text. I think the progress bar refreshes (and goes to foreground) maybe every 0.1s, and the text only when the song changes.
I just tried putting the progress bar into a viewport, but same problem.
It would be great having something that makes the text refresh at the same time as the progress bar.
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EDIT :
I now found a solution, not really clean, but works. I changed the display of the total time %pt into %?px<%pt|%pt>. I removed the scrolling for the "next song", replaced it by alternatively printing artist and song.
Here is the new .wps :
%wd
%X|back.bmp|
%xl|a|battery.bmp|5|5|4|
%xl|b|volume.bmp|127|14|5|
%xl|c|playMode.bmp|8|195|5|
%xl|d|repeat.bmp|27|195|4|
%?bl<||%xdaa|%xdab|%xdac|%xdad>
%?pv<|%xdba|%xdbb|%xdbc|%xdbd|%xdbe|%xdbe>
%?mp<%xdca|%xdcb|%xdcc|%xdcd|%xdce>
%?mm<|%xdda|%xddb|%xddc|%xddd>
%pb|progressbar.bmp|60|131|90|58|
%Cl|5|30|75|75|
%C
%V|18|2|40|12|-|-|-|
%al%bt
%V|95|2|32|12|-|-|-|
%ar%pvdB
%V|86|50|66|12|-|-|-|
%s%ia
%V|96|73|77|12|-|-|-|
%s%id
%V|34|119|90|12|-|-|-|
%s%it
%V|12|143|115|12|-|-|-|
%Ia;%It
%V|140|119|34|12|-|-|-|
%pr
%V|118|180|34|12|-|-|-|
%?px<%pt|%pt>
%V|40|164|34|12|-|-|-|
%pc
%V|72|205|42|12|-|-|-|
%ck:%cM