From 52054c5d24f50abdb0fc99cebc43f0b25020cb54 Mon Sep 17 00:00:00 2001
From: Misha Kaletsky <15040698+mmkal@users.noreply.github.com>
Date: Wed, 26 Aug 2026 23:42:24 +0100
Subject: [PATCH] video-mode: snap a sliver of selector lead-in forward to the
 first highlight

#40 clamped a selector trim start that landed AFTER a highlight. CI kept
flaking on the mirror case: a start a sliver (36-92ms measured) BEFORE
the first highlight. That sliver is raw footage from the recording's
very first frames, and on a slow runner the screencast starts so late
that those frames already show the action's result - the rendered video
opened on the filled, expanded textarea one frame before the reveal's
empty still (confirmed from the CI run's own raw/rendered artifacts: the
2.0s raw video has text at t=0.04s).

A lead-in shorter than the fill stabilization window (max(0,
timelineOffset) + 3 frames) is not worth keeping; open on the first
highlight instead. Longer lead-ins are untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 src/plugins/video-mode.ts | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/src/plugins/video-mode.ts b/src/plugins/video-mode.ts
index a7956b1..fc5dd3e 100644
--- a/src/plugins/video-mode.ts
+++ b/src/plugins/video-mode.ts
@@ -5612,6 +5612,26 @@ export const videoMode = (options: VideoModeOptions = {}): VideoModePlugin => {
             if (racedHighlightStarts.length > 0) {
               state.sourceRange.start = Math.min(...racedHighlightStarts);
             }
+            // The mirror case: a start a sliver BEFORE the first highlight. That
+            // sliver is raw footage from the recording's very first frames —
+            // exactly the frames a lagging screencast hasn't truthfully
+            // captured (on a slow runner the recorder's first frames can
+            // already show the action's result: a filled field before its
+            // reveal). A lead-in shorter than the fill stabilization window
+            // is not worth keeping; open on the first highlight instead.
+            const firstHighlightStart = Math.min(
+              ...highlights.map((candidate) => candidate.start),
+            );
+            const leadInToleranceMs =
+              Math.max(0, timelineOffset) +
+              rawVideoInfo.frameDurationMs * VIDEO_MODE_FILL_PRE_ACTION_FRAME_PADDING;
+            if (
+              Number.isFinite(firstHighlightStart) &&
+              state.sourceRange.start < firstHighlightStart &&
+              firstHighlightStart - state.sourceRange.start <= leadInToleranceMs
+            ) {
+              state.sourceRange.start = firstHighlightStart;
+            }
           }
           const annotationSourceRange = metadataFor(state).sourceRange;
           const sourceRange = translateSourceRange(annotationSourceRange, timelineOffset);