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For the general issue of interpreting control chars, see #23220 (and duplicate #24572). IDLE is a development environment, not a production environment.
If Tal's hypothesis is correct, a solution for Bernard would be a new addition I want for Run Customized: 'Run with System Terminal'. The idea is that people who know the IDLE editor should be able to develop terminal-specific code in IDLE. Instead of Save, open or switch to Terminal/Command Prompt, enter 'python -i path/to/file.py, switch back to editor, one could hit Shift-F5, click [ ] Run with Terminal, click [OK]. The use cases are use of functions like Windows-only kbhit() and interpretation of output chars as on the default system console/terminal. The resolution for this issue should be that I open an issue so that others can contribute to doing this.
As for what is happening with the keras output: The block character appears to be the Inverse Bullet character. Here are 1 and 3: '◘ ◘◘◘'. In tk Text on Windows, the height and width depends on the font. In some, it is twice tall as wide, as at https://www.compart.com/en/unicode/U+25D8.
Here are 1 and 3 \b backspaces: ' '. Firefox displays each as a square box with 0 0 0 8. On Windows, tk displays \b and many other control chars as a White Vertical Rectangle, https://www.compart.com/en/unicode/U+25af. Here are 1 and 3: '▯ ▯▯▯', except that when representing backspaces, there is not the wide space between.
In the png, the printable ascii runs overlap by 7 while the blob runs overlap by 6. If the blobs were \bs, there would seem to be 1 too few. Until Bernard copies and pastes the 'normal' output, or exactly describes it, if dynamic, and copies and pastes the 'wrong' tk output, I don't want to speculate further on what he saw.