Pipeline Graph Structural Update: Small UI Fixes

What does this MR do?

Fixes UI bugs listed in the table.

Screenshots (strongly suggested)

	before	after
fix scrolling within sub-graphs
dropdown appear on top
click upstream button when open

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug](https://docs.gitlab.com/ee/development/testing_guide/index.html). Consider all test levels. See the Test Planning Process.

Related to #276949 (closed)