Fix scroll preservation for customized container scrollbars.

init.lua

@@ -752,6 +752,7 @@ local fs_scrollbar = {
         return e
     end,
     onchange = function(e, fn)
+        e._has_onchange = true
         ctx._events.on_scrollbar_event[e.__id] = fn
         return e
     end,
@@ -805,9 +806,11 @@ local fs_scroll_container = {
         end
         out[#out +1] = "scroll_container_end[]"
         
+        local v = e.__fs._ctx.fields[e.__id]
         if not e._scrollbar then
-            local v = e.__fs._ctx.fields[e.__id]
             out[#out +1] = string.format("scrollbar[-800,-800;0,0;%s;%s;%s]", get(e, "orientation"), e.__id, v and v:sub(5) or "")
+        elseif not e._scrollbar._has_onchange and v then
+            e._scrollbar.value = v:sub(5)
         end
         
         return table.concat(out)
@@ -843,7 +846,8 @@ local fs_scroll_container = {
             e._scrollbar:onchange(fn)
             return e
         end
-        e._scrollbar = fs_scrollbar(-800, -800, 0, 0, e._orientation, e.__id):onchange(fn)
+        e._scrollbar = fs_scrollbar(-800, -800, 0, 0, e._orientation):onchange(fn)
+        e._scrollbar.__id = e.__id
         return e
     end,
     -- A non-transient name is required in order to preserve scroll position upon a rebuild.
@@ -1479,6 +1483,7 @@ imfs = {
         local container = {__parent = ctx, _events = setmetatable({}, {__index = ctx._events, __newindex = ctx._events}), __fs = ctx.__fs or ctx}
         table.insert(ctx, container)
         ctx = container
+        return container
     end,
     container_end = function()
         ctx = ctx.__parent