MultipleImageStrategy::update() and handleClick() ran util::command::exec
(a blocking fork+exec+read) on the GTK main thread, so the whole bar froze
for the script's duration on every interval and on every click.
Move the exec into a new IStrategy::fetch() hook that the SleeperThread
worker runs before dp.emit(); update() now only parses the cached output
and draws on the main thread (mirroring how custom.cpp separates exec from
formatting). handleClick() uses forkExec() so clicks fire-and-forget instead
of blocking on the command's output. The entries and single-image paths are
unchanged.
Alex