File selection text field does not feel like a fully featured text input

This is just a polishing issue but interacting with the open file text field that appears when you click on the name of the open file doesn't feel as natural as it could when interacting with text fields with a mouse and keyboard. I experienced this using the web based DevTools. I'm not sure what the interactions are like on the version built for desktop use.

Mainly, it doesn't support text highlighting. There are multiple ways that I usually expect text inputs to allow for selection:
* Click and drag with the mouse doesn't highlight any text as expected.
* Triple click doesn't highlight all the text in the field as expected.
* Holding shift while moving the cursor with the arrow keys doesn't highlight the text between the original and new cursor locations as expected.
* Holding shift while moving the cursor with the home/end keys doesn't highlight the text between the original and new cursor locations as expected.

If I could highlight text I also would expect that next text entry to replace all the highlighted text.

You should also be able to copy highlighted text to the clipboard.

Start by reproducing the behavior in the web-based DevTools open-file text field with a mouse and keyboard. Check the four described selection interactions, replacement of highlighted text, and clipboard copying; done means these text-input behaviors work as expected.

索引モデルが issue の本文から書いたものです。