Utilise the unused gap between sidebar and editor, in centered layout mode

[vicspidy]

In centered layout mode, when the sidebar is unhidden, it currently shrinks the editor panel width even if there is enough space for the sidebar to expand without affecting the editor size. Ideally, the sidebar should be allowed to expand fully until no more space remains. Only then should it start pushing the editor panel to the right, and if the available space is completely used up, it should then begin reducing the editor panel's width.

Similar to: #24612