Severity: Critical
InDesign Version: 21.2 x64
Language: Arabic (RTL)

Bug Summary

Incorrect positioning of Arabic diacritics (Kasra + Shadda) when ligatures are enabled.
Input order of combining marks is ignored, breaking previous behavior (pre-v17).

Environment

Text Direction: RTL

Language: Arabic

Font: KFGQPC Uthman Taha Naskh

Composer: World-Ready Composer

Ligatures: Enabled

Diacritic Positioning panel tested (no effect)

Description

In versions up to 17, manual input order of Arabic diacritics controlled stacking:

Kasra (U+0650) then Shadda (U+0651) → Kasra under base letter

Shadda then Kasra → Kasra under Shadda

In version 21.2 x64, InDesign enforces mark-to-mark positioning strictly.
Kasra always attaches under Shadda, regardless of input order.

This removes manual typographic control required in professional Arabic publishing.

Not a Font Issue

Font tested: KFGQPC Uthman Taha Naskh

Works correctly in Microsoft Word

Word respects input order

Therefore issue originates from InDesign shaping engine, not the font

Workarounds Tested
1. Disable Ligatures

Result: Kasra appears under base letter (correct)
Problem: Not viable for production — breaks Arabic typography and workflow in large documents (800+ pages)

1. Diacritic Positioning Panel

No effect on stacking behavior

1. GREP + Baseline Shift

Only visual workaround — not a shaping fix

Additional Observation

If entering:
Kasra + Shadda + Kasra

The Kasra appears under the base letter (duplicated), indicating partial internal support for alternate stacking.

Steps to Reproduce

Set paragraph direction to RTL

Use font: KFGQPC Uthman Taha Naskh

Enable Ligatures

Type a letter + Kasra + Shadda

Observe: Kasra appears under Shadda

Disable Ligatures

Observe: Kasra appears under base letter

Impact

Breaks classical Arabic typography control

Affects religious and academic publishing

Removes backward compatibility with previous versions

Forces manual verification in large documents

Request

Please investigate:

Interaction between ligatures and mark-to-mark positioning

Changes introduced after v17

Possibility of restoring manual diacritic stacking behavior

Or adding an option such as:
“Respect input order for Arabic diacritics”

Immediate attention requested due to impact on professional Arabic publishing workflows