Please explain recent 'fix' in Commit e73c60e by adding ' > slot' to selector?

Please would you explain why adding ' > slot' fixed Issue: toolbarbuttons_in_tabs_periphery.css makes some tabs jump several by pixels when hovered, resizing of tabs is weird #419?

What underlying change in Firefox 131 made this necessary?

Similarly, why adding ' > slot' to the 'scrollbox[orient="horizontal"] selector' also fixes this old tab-centering code broken by Fx131.0 update?

#tabbrowser-arrowscrollbox:not([overflowing]){ --uc-flex-justify: center; } scrollbox[orient="horizontal"]{ justify-content: var(--uc-flex-justify,initial); }

CSS userstyle from OP YOUBESEENUMBA1's post 'Update 131.0 broke tab-centering code': https://www.reddit.com/r/FirefoxCSS/comments/1fxb0wb/update_1310_broke_tabcentering_code/

(Note. Fedia's codebox not playing... '#tabbrowser' is always displayed as an unwanted circular link to this site?)