Thanks for the PR, Tal.  I've started looking at it and I have just a few initial observations:

1.  Serhiy's msg211068 still happens.  I tried his example of print('a'*1000000).  There was a noticeable delay before the button was created, then a button with 'Squeezed text (12500 lines).' appeared.  I double clicked on it and regretted it.  If providing a paging mechanism (i.e., a 'more' button) is too complicated, I think we still need an alternative way to prevent the user from clicking a button that will hose the system.  

2.  I don't have a middle button.  My mouse has 9 buttons, but none are a middle click and I can't map it on Linux.  :-(   

3.  'preview-command-posix' - I tried 'vim' and 'nano'.  Right-clicking the button opens those editors in the terminal window that I'm running IDLE from, but the text isn't displayed in the window.  When I exit from the editor, I get a traceback:
Warning (from warnings module):
  File "/cpython/Lib/subprocess.py", line 839
    ResourceWarning, source=self)
ResourceWarning: subprocess 2797 is still running


4.  list(range(500)) showed the button (for 30 lines).  I realize that it's configurable, but it seems like a low initial value.  

5.  The context menu option to manually squeeze works nicely, except that the cursor stays next to the button instead of going back to the prompt.

6.  Is it necessary to be able to hide the tooltip?

7.  I know you talked about it already, but I think this may need to be moved from Extensions before merging.  Changing a config value doesn't always immediately apply the change, so it doesn't fit in with how most config works now.  For example, I changed the 'preview' program from a value to nothing.  The option to right-click still existed and right-clicking gave a traceback:
Traceback (most recent call last):
  File "/cpython/Lib/idlelib/squeezer.py", line 181, in preview
    start_new_session=True)
  File "/cpython/Lib/subprocess.py", line 756, in __init__
    restore_signals, start_new_session)
  File "/cpython/Lib/subprocess.py", line 1499, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
PermissionError: [Errno 13] Permission denied: ''

8.  I didn't check the source code, but I wasn't sure what 'Enable editor' did.  I *thought* it would enable squeezing in an editor window by using manual squeeze mode, but it wasn't on the context menu.

9.  Does the button color need to be configurable?  IMHO, the current yellow looks better on Dark than it looks on Classic.  But it does work for both.