If you want to make an extention that has a slider and continiosly updates the image, you will be a little bit surprised.
Photoshop would update the preview image in the Navigator panel (that small preview image, useful for zooming).
But it will not update the main image.
So, basically, user would see the result of the adjustment once he triggers mouse up event.
It could be some weird Photoshop optimization which does not allow canvas updates if an user is drawing - but it is pretty annoying for plugins.
So, ideally, it would be nice to disable this feature if mousedown occures on the plugin. So, keep the old behaviour for Photoshop, but if the user tries to use the slider in the plugin - allow preview for the image.
I could prepare a minimal example for testing with a slider, if you need. Or make a video describing the problem.