Newly created gvim window not focused, existing gvim window is focused instead

Describe the bug
When creating a new gvim window via terminal gvim command, sometimes the new pop-up window doesn't gain focus, instead, the pre-existing gvim window is focused. This causes minor annoyance because user would intuitively think he/she is typing into the new window without checking.

I suspect it has something to do with multi-monitor environment and each monitor has a separate desktop number. Sometimes I typed gvim in the terminal and the new gvim window was opened in another monitor. And in that case often the wrong window was focused. Maybe macvim will focus window instance on the desktop with lowest index number?

Not sure if this is related to #83.

I didn't find out the pattern to reliably reproduce the behavior yet.

To Reproduce
Detailed steps to reproduce the behavior:

1. Open a terminal.
2. Type gvim.
3. A new gvim instance popped up.
4. But the original gvim instance is focused.

Expected behavior
The new gvim instance is focused.

Environment (please complete the following information):

• Vim version: 8.2.2164
• OS: macOS 11.1
• Terminal: iTerm2

まず、macOS 11.1 でターミナルの gvim コマンドを使ってフォーカスの問題を再現します。特に複数のモニターとデスクトップがある場合を確認し、issue #83 と動作を比較してください。新しく作成された gvim ウィンドウが、既存の gvim ウィンドウではなく、一貫してフォーカスを受け取るようになった時点で作業完了です。