Autocomplete can replace text from the document start when no period exists

[MicroMilo]

Summary

insert_completion() uses text_before_cursor.rfind('.') as a cursor position without handling the -1 result. Autocomplete can be triggered for a bare identifier, so a missing period is a reachable input. The code then moves the cursor to position 0 and can replace text from the start of the document instead of replacing the current token.

Code path

• cq_editor/widgets/editor.py:464-478: rfind('.'), cursor positioning, selection, and insertion.
• cq_editor/widgets/editor.py:362: autocomplete is triggered for partial identifiers, not only expressions containing a period.

Steps to reproduce

The source-level boundary is:

text_before_cursor = "import os\nim"
last_period_index = text_before_cursor.rfind(".")
assert last_period_index == -1
assert last_period_index + 1 == 0

In the current implementation, that value is passed to cursor.setPosition() before the completion text is selected and inserted.

Expected behavior

Completion should replace only the current identifier or use an explicit no-period branch. Text before the current token must remain unchanged.

Actual behavior

When no period exists before the cursor, rfind() returns -1; setPosition(0) moves the cursor to the document start and the following selection can delete or replace the first word.

Existing coverage

The checked issue and PR history did not contain an exact report or fix for this cursor calculation.

Suggested fix

Handle the no-period case explicitly, or compute the replacement range from the current token rather than treating -1 + 1 as a valid cursor position.

Suggested tests

• Bare identifier with no period.
• Period at the first character.
• Consecutive periods.
• Existing dotted completion path remains unchanged.

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Submitted with Codex.