If its not on the Window property dropdown list, its not meant to be captured.
Only top level windows are listed. Window capture can be confusing to users because what they see, may not be what they can get.
But lets get a little more technical...
When you bring up OBS' Window capture dialog box, OBS lists only the top level application windows currently running at the time. A top level window is a one that is programmed to have a certain purpose and style in mind. Things like if its a popup window, child window, parent window of child windows, dialog box, etc.
Top level windows do not have parent windows. They are owned by the desktop window manager. That's the criteria OBS is using when filling up the window property list. Visually, they are easy to identify (but not always) for most will have a title, min, max, close buttons. and perhaps a border.
What you are running into is how the application chooses to layout its GUI components within the window. For programs that have more complex GUI layouts, the top level window is the outer container. Objects that are within it, are not top level windows but rather child windows. OBS does not capture child windows like a frame/panel containing other components within it. It make look like a window to you but it isn't.
Hope that makes sense.
