Using separator switches paned layout to proportional

[JanBliznicenko]

Currently, when I use SpPanedLayout and set position as fixed size (not percentage), it keeps the first pane size when resizing window/owner and only the second pane is being resized when owner/window is resized. That is good - how is it intended. For example, when I need left toolbar with fixed width even after window is resized, yet still allow it to be resized manually using separator.

However, once I use the separator (and resize the first pane with fixed size), the layout switches from fixed size of the first pane to proportional (percentual) sizes, so after using separator, the first pane now starts moving when owner/window is resized and no longer acts as a side toolbar.

The separator should honor the original layout (ie. just change the value of fixed size) and not switch to proportional one.