Background

In #76979, batchResizeImage() in packages/vips/src/index.ts generates multiple thumbnail sizes in a single pass after decoding the source image once via copyMemory(). It checks inProgressOperations.has( id ) between thumbnails and breaks on cancel:

for ( const config of resizes ) {
    // Check cancellation between thumbnails.
    if ( ! inProgressOperations.has( id ) ) {
        break;
    }
    // ... generate thumbnail
}

This means a mid-batch cancel abandons any remaining sizes but keeps whatever was already generated up to that point.

Question

What's the expected user experience when a user cancels an upload partway through batch thumbnail generation?

• Atomic: abandon all generated thumbnails and leave no partial state.
• Best-effort: keep the thumbnails already produced and stop processing further sizes (current behavior).
• Complete: ignore cancellation during the batch loop and let it finish.

This was raised as a non-blocker during review of #76979:

Not a blocker, but I'm wondering a tiny bit about what we want to have happen if a cancellation happens between generating thumbnails. Would we want to abandon them all, or ensure completion of thumbnails? (I.e. how atomic do we want to be?) — @andrewserong

Acceptance criteria

• Decide on the intended semantics and document them.
• Update batchResizeImage (and the upload-media store callers) to match the decision.
• Add a test covering the cancel-mid-batch path.