[Annotations] Drawing is wrongly performed on the image if you click anywhere on the sticky header area, when the page is scrolled

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Screenshots v31.0.0 dev

[Affected Platforms]:

• All Windows
• All Mac OS
• All Linux

[Prerequisites]:

• Have the "xpinstall.signatures.required" pref set to "false" and "extensions.legacy.enabled" pref set to "true".
• Have the latest Firefox Screenshots v31.0.0 dev installed from here on a new clean profile.
• Have a long saved shot link.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites and navigate to the saved shot link.
2. Click the "Edit this image" button.
3. Scroll down and click anywhere on the sticky menu.
4. Scroll up the page and observe the image.

[Expected result]:

• No drawing was performed on the image.

[Actual result]:

• Drawing was performed on the image.

[Notes]:

• This issue can be easily noticed if you change the color or if you change the drawing tool.
• This issue is also reproducible if you click the "Crop" tool and the "Save" button.
• Here is a screen recording with the issue:
  

[Softvision-CristinaBadescu]

This issue is no longer reproducible using the latest Firefox Screenshots 31.1.0 dev version, on Windows 10 x64, Mac 10.13.3 and Arch Linux 4.12.