This issue was discovered while reviewing MDL-76261, when I clicked on "Insert" in menu bar and the menu item popup did not show accordingly.

Based on the screenshot and screen recorded video, the menu items are displayed but one them was covered by the header. It would be a good investigation on calculating the distance between the header and tiny editor itself against the height of the menu item container in order to display it under or top of the menu bar.

To reproduce the issue

1. Login as admin
2. Enable the "TinyMCE editor" in site admin > Plugins > Text editors > Manage editors
3. Click the profile > Preferences > Editor preferences
4. Select "TinyMCE editor"
5. Go to profile > edit profile and scroll down to the tiny editor
6. Set the distance between the tiny editor to something similar to the video
7. Click on the "Insert" in menu bar
8. Verify that the top menu, which is "Link" is covered by the header

We will not see this issue in our regular use due to a specific distance between the header and tiny editor, but it's a good chance to have a look in case in the future the page space is just right to display the menu item on the top while it's still being covered by the header