Thank you Yraen for your post. That is what I have done before now. However, on the target window, I would like to keep the window sizeable so that I can hide or show the target's buffs at will.
I came up with a quick but workable hack to fix this: I copied the default EQUI_Templates file into my custom UI folder, and modified the three title bar definition lines for WDT_RoundedNoTitle from:
Now, for every simple-border window that I want to be movable and sizable, I use WDT_RoundedNoCorner as the template and set both sizable and titlebar to true.
With this approach, the user can drag-and-drop the borders to size it and can drag-and-drop a very small area at the top of the window to move it.
Hope this is helpful to others, at least until someone comes up with a better approach.