In the save dialog's checkboxes block, .oo-ui-fieldLayouts have margin-right equal to 1.5em. But the last child doesn't need it since the checkboxes block has padding already:
After the change, the checkboxes in ruwiki won't be split into two lines anymore:
Existing code:
.ve-ui-mwSaveDialog-checkboxes .oo-ui-fieldLayout { display: inline-block; margin: 0 1.5em 0 0; }
Proposed addition:
.ve-ui-mwSaveDialog-checkboxes .oo-ui-fieldLayout:last-child { margin-right: 0; }
(Also, if the checkboxes are split into two lines, the bottom margin is bigger than the top, but I guess a separate task is needed for this.)