When the Okular window is resized, the sidebar maintains its size relative to window size. IE, if I have deliberately changed the sidebar to 200px in width, and I make the window twice as wide, the sidebar will become 400px wide.
This is an undesirable behavior, as the user is likely resizing the window to get a wider view of the rendered PDF contents, and they have already decided that the sidebar is as wide as needed for viewing, e.g., a table of contents.
Instead, the size of the sidebar should be absolute, and not change when the window is resized.