Generate Text on Timeline progressively shifts subtitle timing

[danny20041104]

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OS and Version (please complete the following information):

Operating System: Windows 11 24H2
Shotcut Version: 26.06.25

Describe the bug

Using Generate Text on Timeline progressively shifts the generated text clips away from the original subtitle timing.

The original subtitles displayed through the Subtitle filter are perfectly synchronized with the video. However, after using Generate Text on Timeline, the generated text clips gradually drift over time.

The drift is cumulative. At the beginning of the video the timing is correct, but near the end the generated clips are approximately 400 ms out of sync.

This only affects the generated timeline text. The Subtitle filter itself remains correctly synchronized.
The issue is reproducible with my project.

Steps To Reproduce

Start Shotcut.
Create a new project (tested with both PAL 25 fps and HD 1080p 25 fps).
Import a 25 fps CFR H.264 video.
Add subtitles using the Subtitle filter.
Verify that subtitle timing is correct during playback.
Select Generate Text on Timeline.
Compare the generated text clips with the original subtitle timing.

Expected behavior
The generated text clips should preserve exactly the same timing as the Subtitle filter.
There should be no cumulative timing drift between the generated timeline text and the original subtitles.

Screenshots

[ddennedy]

The issue I see (59.94 fps) is that the subtitles track under the ruler often starts and ends one frame later from the Subtitles item and the text clip on the video track using Generate Text on Timeline. I am not seeing an accumulating error on the text clips; the last text clip starts and ends correctly in a 2:50 minute project.

[ddennedy]

I did not reproduce the reported problem with a 25 fps video but the problem I found still occurs, and I have a fix. Then, when comparing with the Subtitle Burn In video filter I found it has a bug that it has a bug that it displays one frame longer than it should. For example, the item says it ends at 00:03:14.320 in the Subtitles dock, but the Subtitle Burn In filter still shows it at that time plus the frames duration of 40ms at 25 fps. It treats the end time as inclusive, meaning the frame containing that time is included, but SRT semantics treat the end time as exclusive.

[ddennedy]

Fix for the Subtitle Burn In filter made in MLT. Maybe these things fix what you noticed. That's all I could find.