Asynchronous Preview Updates While Dragging/Trimming Strips in the Video Sequencer

Problem

When moving, trimming, or sliding strips in the Video Sequencer, interaction can become sluggish because preview evaluation and frame loading are tied to the transform operation.

Even with options such as "Preview During Transform" disabled, Blender may still need to decode frames, update caches, evaluate effects, or otherwise perform expensive work while the user is dragging. This can make the mouse interaction feel delayed or "sticky", especially with compressed media (H.264/H.265), high-resolution footage, network storage, or complex timelines.

The primary editing action (changing strip positions) and the secondary action (updating the preview image) are currently coupled closely enough that preview evaluation can negatively affect interaction responsiveness.

Proposal

Introduce an optional mode for asynchronous preview updates during strip transforms.

While a strip is being moved, trimmed, or slid:

Prioritize immediate UI responsiveness and strip position updates. Perform preview frame evaluation and decoding in the background. Display the most recently available preview frame if a new one is not yet ready. Update the preview as soon as the requested frame becomes available. Perform a full and accurate evaluation when the transform is confirmed (mouse release).

Conceptually:

During drag: Update strip positions immediately Queue preview requests asynchronously Keep UI responsive Show latest available preview

On release: Perform final evaluation Display exact frame

Benefits

Smoother interaction on large or complex projects. Better usability with highly compressed footage. More responsive editing on lower-end hardware. Consistent editing feel even when media decoding is expensive. Similar behavior to many dedicated NLEs that prioritize interaction latency over immediate preview accuracy during active manipulation.