I would like to make a rule-base labelling depend on: is a label shown in the current view or not. ("Show all labels for this layer (including colliding labels)" is unchecked.)
I would like to use it on a polyline layer, with curved placement.
For example: The default font is 8p. If a label is not shown on the current view (because of colliding labels, I think), then the font size of that label changes to 7p, so it may come to be shown. If it does't, the font size of it changes to 6p, etc.