rectFor

pub fn rectFor(self: *BasicLayout, contentRect: Rect, id: Id, min_size: Size, e: Options.Expand, g: Options.Gravity) Rect {
        if (self.seen_expanded) {
            // A single vertically expanded child can take the rest of the
            // space, but it should be the last (usually only) child.
            //
            // Here we have a child after an expanded one, so it will get no space.
            //
            // If you want that to work, wrap the children in a vertical box.
            const cw = dvui.currentWindow();
            dvui.log.err("{s}:{d} rectFor() got child {x} after expanded child", .{ @src().file, @src().line, id });
            var iter = cw.current_parent.data().iterator();
            while (iter.next()) |wd| {
                dvui.log.debug("  {s}:{d} {s} {x}", .{
                    wd.src.file,
                    wd.src.line,
                    wd.options.name orelse "???",
                    wd.id,
                });
                dvui.Debug.errorOutline(wd.rectScale().r);
            }
        }

        var r = contentRect;

        switch (self.dir) {
            .vertical => {
                if (e.isVertical()) {
                    self.seen_expanded = true;
                }
                r.y += self.pos;
                r.h = @max(0, r.h - self.pos);
            },
            .horizontal => {
                if (e.isHorizontal()) {
                    self.seen_expanded = true;
                }
                r.x += self.pos;
                r.w = @max(0, r.w - self.pos);
            },
        }

        const ret = dvui.placeIn(r, min_size, e, g);

        switch (self.dir) {
            .vertical => self.pos += ret.h,
            .horizontal => self.pos += ret.w,
        }

        return ret;
    }