"Tab Split" button is not correctly displayed on other windows that were already opened before installing the add-on

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Tab Split 1.0a5

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Have the "xpinstall.signatures.required" pref set to "false".
• Have the "extensions.legacy.enabled" pref set to "true".
• Have 2 windows open.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Install the latest "Tab Split" version through "about:debugging" page.
3. Switch to the other window and observe its toolbar.

[Expected result]:

• A new button named "Tab Split" appears in the toolbar.

[Actual result]:

• An empty space is displayed instead of the "Tab Split" button.

[Notes]:

• When hovering the empty button, a gray rectangle appears.
• Here is a screen recording with this issue:

[fzzzy]

I believe this is fixed now -- please re-open if not.

[Softvision-CristinaBadescu]

This issue is no longer reproducible using Tab Split v0.1.7, on latest Nightly build 60.0a1 (2018-02-09), on Windows 10 x64, Mac 10.12.6 and Ubuntu 14.04 x64.

[Softvision-CristinaBadescu]

I'm going to reopen this issue since it is reproducible using Tab Split v0.1.8, on latest Nightly build 60.0a1 (2018-02-16), on Windows 10 x64 and Mac 10.13.3.

This issue also occurs if you open other windows after installation.