Whenever in doubt, see me.
When in doubt, see me.
Wherever possible, all moving parts should be tested.
Where possible, all moving parts should be tested.

1. ever 跟疑问代词、疑问副词连用，表示强调。
When [ Where, How ] ever did you lose it?
你到底何时 [ 何地, 怎样 ] 丢失了这个东西?
——《现代英汉综合大词典》
Why ever didn't you tell me?
你究竟为何不告诉我？
《柯林斯词典》

其他词都可连写：whenever, wherever, whatever, however 等，why ever 分开写。
2. 跟从属连词连用，除了强调外，还有可能变成让步从句。即网友的句子。

The cardboard is light-proof where thick.
本句改为 wherever 也是可以的，仍为强调。比较：
The cardboard is light-proof where thick.
这种纸板，厚的地方是不透光的。
The cardboard is light-proof wherever thick.
这种纸板，在任何厚的地方都不透光。