Add a new option to the Vertical Justification settings for text frames: "Conditional Justify".
Behavior

When Conditional Justify is selected, vertical justification is applied only when the text frame is sufficiently filled.
Add a numeric field: Content fill threshold (%).
    If the frame’s text occupies a percentage equal to or greater than this threshold, the frame receives full vertical justification.
    If the frame’s fill is below this threshold, the frame uses the chosen fallback alignment.

Fallback option

Provide a dropdown: Fallback alignment with the classic vertical choices: Top, Center, Bottom.

Notes / Suggested defaults

Default Content fill threshold: 90% (editable).
Default Fallback alignment: Top.
Show a small live preview or indicator in the Text Frame Options dialog that displays the current frame’s content fill percentage.

Rationale (brief)

This lets designers apply full vertical justification only when a frame is nearly full, avoiding awkward spacing in partially filled frames while maintaining consistent alignment when content is dense.
Additional conditions may be added later (e.g., minimum/maximum number of paragraphs, presence of headings, or specific glyph counts).