IME で開くウィンドウに使用される現在のテキスト入力の位置
Some language IMEs such as Japanese will open windows while the user is typing text, to aid the user in picking the correct input strings. These windows are expected to pop up at the current cursor position, so the IME needs to know where input is displayed. When using Unity's built in GUI system for text input, Unity will take care of setting the cursor position for the IME. However, if you wish to implement your own GUI for text input, you need to set this to the current text input position for IME windows to show up correctly. Additional resources: Input.imeCompositionMode, Input.compositionString.