Output blocks after 65k lines when using run_in_terminal

Hey all,

When using run_in_terminal, the output will block forever after printing 65536 lines.
This is using the latest prompt_toolkit version (2.0.8), and python 3.

To reproduce simply run the this_blocks_after_65k_lines method:

def this_blocks_after_65k_lines():
    i = 0
    while True:
        run_in_terminal(lambda: print_formatted_text(FormattedText([('', f'{i}')])))
        i = i + 1

def this_does_not_block():
    i = 0
    while True:
        print_formatted_text(FormattedText([('', f'{i}')]))
        i = i + 1

The output will keep scrolling numbers until 65536, then it stops.

Start by running the reported this_blocks_after_65k_lines reproduction with Python 3 and prompt_toolkit 2.0.8, comparing it with this_does_not_block. Trace the run_in_terminal path and determine why output stops at 65536 lines; done means the reproducer continues printing beyond that point without blocking.