You can lock the layer or filter node you are working on and move to another filter in the Style Editor to change its parameters and immediately see the preview, without the need to switch between the two Properties + Preview panes.

Lock the Preview

Select the preview you want to keep visible. Click the Lock icon to lock the preview (e.g. in our case we choose to keep Layer #1 visible).

Select the filter node in the graph that you want to change (e.g. Gain Curve) and change its properties in the Properties + Preview pane. The preview you have locked previously will now be rendered in real time (Layer #1).

Select another filter node in the graph that you want to change (e.g. Random Painter) and change its properties in the Properties + Preview pane. The preview you have locked previously will now be rendered in real time (Layer #1) and a Progress Bar will show the elapsed processing time.

Other results are possible without switching between the Compositing and Properties+Preview pane.