Thank you @pizzafilms for this summary. We should make it easier to use a custom version of the StandaloneFilterWindow.
For many, it may be easier to just roll your own Application and Window. Luckily doing this is just as easy as creating any standalone GUI app with JUCE. Just create your GUI app as you would any other JUCE standalone GUI app but add JUCE_USE_CUSTOM_PLUGIN_STANDALONE_APP=1 to the list of preprocessor defines in the Projucer.
You can find more info my post here (“option 2”).