[Annotations] Writing in the textbox wrongly moves it to the left

[Softvision-PatriciuPop]

[Affected versions]:

• Nightly 63.0a1
• Dev Edition 62.0b7
• Release Unbranded

[Affected Platforms]:

• All Windows
• All Linux
• All Mac

[Prerequisites]:

• The "xpinstall.signatures.required" boolean pref is set to "false".
• The "extensions.legacy.enabled" boolean pref is set to "true".
• The "extensions.install.requireBuiltInCerts " boolean pref is set to "false"
• Have the Screenshots 33.0.0 dev version installed from here, on a new clean profile.

[Steps to reproduce]:

1. Open the profile from prerequisites and navigate to a website (eg. www.wikipedia.com).
2. Click the "Edit this image" button.
3. Click the "Add text" button and write a long string of text.
4. Press and hold the "Backspace" key and observe the behavior.

[Expected results]:

• The textbox remains in a fixed position.

[Actual results]:

• The textbox moves to the left as the characters are removed.

[Notes]:

• If the textbox is moved to the left edge of the image, writing in it will cause the textbox to disappear from view.
• I have attached a screencast of the issue:
  

[punamdahiya]

@Softvision-PatriciuPop with #4658 fix in master, can you please check if this issue is replicable following above steps

[Softvision-PatriciuPop]

This issue is still reproducible using the latest Firefox Screenshots dev version, on latest Nightly build 63.0a1. Verified on Windows 10 x64, Mac 10.13.5 and Ubuntu 16.04.

[chenba]

Currently as the user delete characters, an equal amount of width is remove from both end of the input element regardless of where the input is relative to the shot image. As a user enter text, the input is shifted to the left so that the cursor can stay in view on the right side of the shot (this is a default browser behavior). The effect of those two things is this bug.

[Softvision-CristinaBadescu]

This issue is no longer reproducible using the latest Firefox Screenshots dev version, on latest Nightly build 63.0a1. Verified on Windows 10 x64, Mac 10.13.5 and Ubuntu 16.04 x64.