Try this space-style for a fixed 600px TreeView SB-Panel
.sb-panel {
flex: 0 0 600px !important;
}
or 30% for a 30/70% ratio
.sb-panel {
flex: 0 0 30% !important;
}
FYI this will resize all SB-Panels not just the Treeview, but there aren’t many other plugs which use sb-panel, so you should be safe if you don’t use other plugs.
yes, right-clicked on the Treeview Panel then Inspect Element then in the Developer Tools i checked which is the topmost div selector’s class of the Treeview panel, and there it was: sp-panel that is how I usually do the space-style restylings to override the default values.