PPTX preview renders overlapping header shapes above titles despite correct PowerPoint file

Summary

The Codex desktop app preview can render PowerPoint slides with an incorrect z-order or clipping relationship between decorative header shapes and slide titles.

In the affected deck, three overlapping teal rounded rectangles are positioned partially outside the top of the slide and are intended to remain behind the title. In the in-app preview, these shapes appear above the title and obscure portions of the text. The same PPTX opens and renders correctly in Microsoft PowerPoint.

Steps to reproduce

1. Open or attach a PPTX containing multiple overlapping rounded rectangles in the header.
2. Position the rectangles partially outside the top edge of the slide and behind a title textbox.
3. View the slide through the PPTX preview inside the Codex desktop app.
4. Compare the preview with the same file opened in Microsoft PowerPoint.

Expected behavior

The preview should preserve the PowerPoint file's z-order, clipping, position, and grouping so the decorative shapes remain behind the title.

Actual behavior

The header shapes render above the title and partially hide it, even though the PPTX itself is correct when opened in PowerPoint.

Impact

• The preview makes correct PPTX files appear visually corrupted.
• It reduces confidence in visual QA performed inside the app.
• It can lead users or agents to “fix” a deck that does not need changes, potentially altering protected brand elements.
• The issue is especially visible in branded templates that use overlapping or partially off-canvas header shapes.

Environment

• Codex/ChatGPT desktop app: 26.707.91948 (build 5440)
• macOS: 26.5.2 (build 25F84)

No client PPTX or screenshots are attached because the available reproduction contains confidential client material. A sanitized reproduction can be created if needed.