I experimented more with 3.5.1, Win 10 and formulated a question and hypothesis.

In a text widget, pressing button 1 (usually the left) moves the cursor to the nearest between-character slice line.  Hold the button down allows selecting text.  Releasing and pressing again selects a word after moving the cursor.  Pretty standard behavior, independent of IDLE.  I believe the bindings are Text class rather than instance bindings.

When a completion box is present, button 1 down outside the box dismiss the box and moves the cursor.  I believe this is intended.  I don't know how general this behavior is.

The IDLE completion box binds the button *release* to, I believe, "move cursor to line clicked and display line in text".  A double click is supposed to do something more.  So the button1 press event goes to the Text, which dismisses the popup and moves the cursor.

Question: does the text widget freezes on button down or not until button up.  The experiment is to press and hold the button while moving the mouse.  Does the cursor move and select text while the button is down, or does it freeze immediately.  Paul, could you try this?

Hypothesis (cause): a delicate timing issue? Is the button-up event somehow 'half-caught'?

Hypothesis (fix): prevent the text from seeing or acting on the button down event.  It should be possible to have the completion box catch the down event first and return 'break'.  My first attempt failed but I was too tired to continue.