"u" operation sometimes acts on a window that isn't focused

[Biacco42]

I'm opening this issue because:

• I'll report a bug
• I'll propose a new feature

Description

"u" operation sometimes acts on a window that isn't focused.
However, the following command "u" may act on the focused window or maybe another not focused window.

Failure Information (for bugs)

Steps to Reproduce

1. Open some windows
2. Close tabs on some windows (in any shortcuts or GUI)
3. Press "u" key to operate "reopen closed tab"

System configuration

• Operating system: Windows 10 Home ver. 1703 build 15063.674
• Firefox version: Firefox Quantum (57.0b9)
• Vim-Vixen version: 0.1

Console logs

Any relevant log in developer tools: