Render faux caret during indentation animation

[raineorshine]

Steps to Reproduce

- a
  - b
    - Hello World
      - c

1. Set the cursor on b.
2. Tap b to open the keyboard.
3. Tap in between Hello and World.

Current Behavior

The native browser caret is invisible during the animation as expected. This solves #4, but is particularly disorienting when tapping somewhere in the middle of a thought to place the caret. The user does not get the immediate visual feedback of whether they placed the caret at the desired offset or not.

Expected Behavior

Render a faux caret where the native browser caret should be while it is invisible.

Hide the faux caret as soon as the native caret is visible again, or if the user begins interacting with the thought, i.e.:

1. When the indentation animation ends.
2. When the user starts typing.
3. When the browser selection changes.