Issue with resizable scroll containers

[lnm95]

I encountered an issue where a scroll container that changes its size ends up with an incorrect _base_offset calculation.

Minimal reproduction steps:

• Enable visibility
• Resize the container so it becomes scrollable
• Scroll down to change pos
• Disable visibility
• Increase the container size so it is no longer scrollable
• Enable visibility and then disable it again
• Reduce the size
• Enable visibility

At this point, the bug occurs: content_node.position is now zero, but pos is still preserved.

As far as I was able to determine, this happens in _update_content_margins() when computing _base_offset = ScrollLayout.calculate_base_offset(content_node, pos)

As a temporary workaround, I completely replaced ScrollLayout.calculate_base_offset() with ScrollLayout.calculate_initial_offset(). This fixed the issue for me, but I’m not sure what side effects this change might have.