I don't know the best place to leave feedback, which reaches the developers, so I try it here.

I think the new "window rules" dialog got much worse than it was before. I am not sure why it was changed at all, as it worked without any problems before.

Now it does not match the rest of the KDE UI and is confusing to use.

- The breadcrumbs navigation looks a bit off, but especially it does not navigate but just changes the focus from the rules list to the edit dialog and back. The font seems too large to match the UI. It has no visual clue that you can actually click it.
- There is no indication which item is opened in the right pane.
- The rules list looks okay. It didn't change much, and I guess there is not much you can do wrong there. It's a bit strange not to have a list widget but a custom implementation there, but that's no big deal.
- Clicking an item in the list does not influence the right pane at all. You need to click the edit button, even when the whole list item is highlighted on mouseover. Clicking the text shows a caret and allows you to edit the title without providing a visual clue that you are in edit mode (e.g., changing the list item to a text field). Also, there is no need for an edit function if you associate the whole item with the edit button, which allows you to edit the title in the right pane.
- The Add properties dialog is confusing. It isn't a dialog at all but somehow scrolls up an overlay in the right pane. When it was straightforward to find out which properties match a window, it now looks really confusing. It is hard to find out what the items there are doing.
- When you click a plus icon, the list item vanished without any indication of where it is now. You find them only when you close the overlay. I would expect that I click the plus icon and then can edit the property. So it seems more like I click the plus icon, and it acts like a minus (i.e., removing a property from the list). Also, it looks like all items in the list (except the removed ones) are active, not vice versa. A three-pane view with the typical move item between active <-> inactive list would be a much better UI.
- There is no clear distinction between properties for matching a window and properties to define a rule. So you are clicking items from an extensive list with a few headers and do not know what is matching and what is enforcing rules.
- The default for all added items is "no effect." This is an unexpected default because I just added them to the list, so I probably want them to be active. Maybe the plus button should open a context menu to select "enforce, enforce temporarily, initialy, ..." before adding them. This would also simplify seeing what is a rule and what is an action
- Clicking "Add new ..." creates an item in the list, even when the user does not click any "save" button. As the list has little contrast and no highlighting which item is active, you don't instantly see that it is already added to the list and search for the save button.
- Experimenting with the new UI, I now have 5 "New window settings" items in the list because I switched between existing rules and "Add new ..." back and forth, unintentionally adding a new empty rule each time.
- "Add new ..." has three dots, which indicate a popup window or another dialog requiring confirmation before the action is executed. When the rule is added instantly, the three dots should probably be removed.
- "Match whole windows class [much horizontal space] ( ) yes  (x) no" looks like there is something missing. It does not indicate what should be matched if you don't know what the dialog is doing. This would be more something that should be a checkbox to the left or right of the "Window class ..." item.
- When having the window too small horizontally, the items are abbreviated with three dots instead of showing a horizontal scroll bar. This again violates the UI metaphor that three dots indicate a further action or that there is more that belongs to the item.
- Making the Window even smaller horizontally, the whole list is collapsed and only reachable via the breadcrumbs and left/right arrows.
- The left/right arrows are only shown when the list is collapsed, making the header inconsistent between different window sizes.
- Before clicking the edit button on an item in the list, there is only one pane. After clicking an item, there are two panes without an obvious option to close the right pane again, restoring the list's old state.
- Clicking export shows additional checkboxes in the list, which were not visible before as well. It also shows an extra header in the left pane. A more common UI would be to have a multi-select list instead of the checkboxes and export the selected items. Another option would be to open a popup dialog with a multi-select list or a list with checkboxes.

The whole part does not match the rest of the "configure window manager" dialog at all. It looks like being from a mobile website instead of belonging to a UI. It also has many inconsistencies in its UI metaphors and common and established UI patterns used in most KDE applications.

I really dislike the change, and I wonder why it was necessary. When I should say what I would like for an improvement, I would just wish to have the old UI, which worked well for me, back.