bug: Black frame appears at end of rendered video (FFmpeg/editly sizing issue)

[SecurityQQ]

Bug Report

Description

Some rendered videos have a black frame at the end. This appears to be a duration/sizing mismatch in the FFmpeg pipeline (editly layer) — when the source clip doesn't perfectly fill the expected duration, a black frame gets appended to pad the remaining time.

Reproduction

Example video with black last frame:
https://su.varg.ai/renders/1772764472485_yyzfle0c.mp4

Root cause (needs investigation)

Likely one of:

1. Editly clip duration rounding — the clip duration in the editly spec is slightly longer than the actual source video, causing FFmpeg to pad with black
2. FFmpeg trim filter — cutTo or duration calculation results in a fractional-frame overshoot
3. Concat demuxer — when concatenating clips, a duration mismatch between the editly spec and the actual media causes a black frame gap
4. FPS mismatch — source video FPS doesn't divide evenly into the target duration, leaving a partial frame that renders as black

Where to investigate

• templates/sdk/src/ai-sdk/providers/editly/index.ts — clip duration calculation (cutTo - cutFrom), trim filters, concat logic
• FFmpeg filter chain construction — check if -shortest or padding flags are involved
• Check if tpad or similar filter is being applied implicitly

Expected behavior

Videos should end cleanly on the last real frame with no black padding. If a source clip is slightly shorter than the specified duration, the last frame should be held (freeze frame) rather than showing black.

Suggested fix directions

• Add -shortest flag to FFmpeg output if not already present
• Clamp clip durations to actual source media duration
• Use tpad=stop_mode=clone to freeze the last frame instead of showing black
• Investigate rounding strategy for frame-accurate trimming