tagging - speed with tag-collection and detach/attach

In tagging.c we have:

static void _raise_signal_tag_changed(dt_lib_module_t *self)
{
  dt_lib_tagging_t *d = self->data;
  // when collection is on tag any attach & detach becomes very slow
  // speeding up when jumping from tag collection to the other
  // the cost is that tag collection doesn't reflect the tag changes real time
  if(!d->collection[0])
  {
    // raises change only for other modules

And indeed when creating a tag-collection from tagging module using the contextual menu "go to tag collection" attaching or detaching a tag does not update the count of image for each tag in the collection module. Why not.

But if a tag collection is selected directly from the collection module itself the count update when attaching/detaching a tag is made and I have never experienced an unusable slowdown.

My proposal is that after 5.4 release the condition above is removed.

Start with tagging.c and inspect _raise_signal_tag_changed(), especially the collection condition described in the issue. Reproduce tag attach and detach through both the tagging module's "go to tag collection" path and a collection selected directly; done means tag counts update correctly without the reported slowdown.