Popover position is off on Safari with a Classic theme and when using the keyboard

[afercia]

Description

Noticed while testing #42187

This appears to happen only with Safari and only when:

• The active theme is a classic theme.
• Only on some popovers, when using the keyboard.

I'm not sure what the root cause is. From a quick look, I'm not sure the popover calculation takes into account the case when the post editor has no iframe (with a Classic theme).

Step-by-step reproduction instructions

• Use Safari.
• Activate a Classic theme e.g. Twenty Twenty-One.
• Go to the post editor ad click on a paragraph.
• Press Shift+Tab to navigate to the block toolbar.
• Use the right/left arrow keys to navigate to the 'More' button and press Enter to open the dropdown menu.
• The first menu item is 'Highlight' and is already selected: just press Enter again.
• A color picker popover opens.
• Observe the popover is mispositioned and aligned to the left edge of the screen. Also the vertical position appears to be off.
• Refresh the page.
• This time, open the 'Highlight' popover by using the mouse.
• Observe the popover is correctly positioned.
• Activate a block-based theme e.g. Twenty Twenty-Three.
• Repeat the steps above.
• Observe the popover is always correctly positioned.

Screenshots, screen recording, code snippet

Mispositioned popover: Safari, classic theme, using the keyboard:

Correct popover position: Safari, block-based theme, using the keyboard:

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes