Hi @robvp, I agree with you that this is not ideal behaviour but sadly one of the pains of having “modal dialogs”. This though is an area where I believe the KNIME modern UI is gradually improving, and if you read on you’ll see that in the latest KNIME release there is a configuration mode that is moving away from modal windows, to a configuration panel.
Which version of KNIME are you using?
Below is a post I wrote away from the public area of the forum, during our white-hat testing (community hacking days) for the KNIME 5.4 release, but I’m reposting/pasting it below as I think it is a move towards what you are discussing which you may find helpful.
WARNING: You may wish your audio to be turned down if in open office. There is generally no audio on the the following videos, except that I now realise it recorded the windows “chime” when I attempt to access a disabled window, and you may find that annoying if your volume is up!
From Community Hacking Days event for KNIME AP 5.4
I just realised that in 5.4, detached data windows now stay on top of the workflow even when clicking on other nodes. Thanks KNIME team. That is great, and I hope that it is intentional and will be retained in this and future releases. 
By way of context…
In 5.3.3, if I open a detached popup data window and click elsewhere, it goes behind the KNIME main application window. If I open a node configuration, I cannot bring the detached window to foreground and so cannot read the data during node config, unless I have taken steps to put it on a separate monitor
In 5.4 it stays on top. If using (the normal) “popup window” config dialogs, the detached data window is not active, because the popup is modal, so cannot be interacted with. So an improvement, but not perfect.
BUT… and this to me is something that is particularly useful… if we set KNIME to use the new “embedded inside application” config dialogs, the popup window remains interactive, provided the node being configured is modern UI, which means, finally we can copy and paste data from the data window into, say, the Row Filter. To me this is a big improvement.
Attachments:
KNIME 5.3.3
This demonstrates what happens if I try to view data in a popup table whilst configuring a node:
KNIME 5.4 With “popup config windows”
Here the popup data window remains visible, which is an improvement on 5.3, but is still inactive.
KNIME 5.4 With “embedded config windows”
When the node being configured has a Modern UI, this is where it gets really useful, as not only does the popup data window remain visible, but it continues to work interactively as the config window is no longer modal.
In the video,I use the Ctrl-C / Ctrl-V in Windows to copy and paste from the data table to the Row Filter. In other OS, I’m assuming the respective keyboard shortcuts also work.
I hope that is of interest.
If you are not currently using KNIME 5.4, I would recommend waiting for KNIME 5.4.1 (although I don’t know timescales) rather than switching any critical systems to 5.4. This does hopefully give you something to look forward to though, or try out on a side-by-side installation. 
