This has been hanging around since 2.8.x days with others here:
No one ever really saw a resolution.
It's not my plugins, they're deactivated.
On posts and pages you cannot pull down the Screen Options, nor any of the Media buttons. With or without the GUI interface.
Upgraded to 2.9 this morning and the issue is still present.
On other screens like the dashboard things are working as normal (screen options drop down, can move dialogs etc)
Looking for new solutions, and again, this isn't about a conflicting plugin or about the media uploader stalling. This is about media dialog never even launching.