fix(captions): pick the closest position preset, not the first one checked

activeHorizontalPositionPreset checked "is this centered?" before "is this
flush left/right?" — fine normally, but as width approaches 100 the whole
reachable range shrinks toward 0 right along with it, so a band sitting
exactly at the true left/right edge could fall inside the center check's
epsilon too and get reported as centered. Comparing all three candidates and
keeping the nearest one is correct regardless of how narrow the range gets.

Found via code review (CodeRabbit hit its OSS rate limit on the PR, so this
ran as a subagent review instead). Reachable today only through a hand-edited
or externally-generated project file — legacyEditor.captions has no schema
validation — not through the shipped integer-stepped width slider.

Author: EtienneLescot

src/lib/ai-edition/captions/captions.test.ts

@@ -6,6 +6,7 @@ import {
 	activeHorizontalPositionPreset,
 	activeVerticalPositionPreset,
 	CAPTION_BAND_HEIGHT_PCT,
+	CAPTION_POSITION_PRESET_EPSILON,
 	captionBackgroundCss,
 	captionBandRect,
 	captionHorizontalPositionOffset,
@@ -277,6 +278,18 @@ describe("caption position presets", () => {
 		expect(activeHorizontalPositionPreset({ ...ON, width: 100, offsetX: 0 })).toBe("center");
 	});
 
+	it("still picks left/right over center when a near-full-width band squeezes them inside the epsilon", () => {
+		// At width this close to 100, range.x.min/max themselves fall inside
+		// CAPTION_POSITION_PRESET_EPSILON of 0 — checking "is this near center?"
+		// first would wrongly claim an offset that is exactly at the true edge.
+		const squeezed = { ...ON, width: 100 - 1e-7 };
+		const range = captionOffsetRange(squeezed);
+		expect(Math.abs(range.x.min)).toBeLessThan(CAPTION_POSITION_PRESET_EPSILON);
+		expect(activeHorizontalPositionPreset({ ...squeezed, offsetX: range.x.min })).toBe("left");
+		expect(activeHorizontalPositionPreset({ ...squeezed, offsetX: range.x.max })).toBe("right");
+		expect(activeHorizontalPositionPreset({ ...squeezed, offsetX: 0 })).toBe("center");
+	});
+
 	it("sets offsetX to the true frame edge for left/right, matching the reachable range", () => {
 		const range = captionOffsetRange(ON);
 		expect(captionHorizontalPositionOffset(ON, "left")).toBeCloseTo(range.x.min, 6);

src/lib/ai-edition/captions/settings.ts

@@ -211,18 +211,33 @@ export function activeVerticalPositionPreset(
  * There is no stored `horizontalPosition` field — `offsetX` is already an
  * absolute-feeling value centred on 0 with a range that reaches both frame
  * edges (see `captionOffsetRange`), so "left"/"center"/"right" are just names
- * for three points on that existing range, checked in this order because all
- * three coincide at `offsetX === 0` when the band is full-width (no travel).
+ * for three points on that existing range. All three coincide at `offsetX===0`
+ * when the band is full-width (no travel) — and can also *nearly* coincide
+ * for a band merely close to full-width, where `range.x.min`/`max` shrink
+ * toward 0 as well. Picking the CLOSEST candidate (not the first one within
+ * epsilon) is what keeps that near-degenerate case from reporting "center"
+ * for an offset that is actually sitting exactly on `range.x.min`/`max`.
  */
 export function activeHorizontalPositionPreset(
 	settings: CaptionSettings,
 ): CaptionHorizontalPosition | null {
 	const range = captionOffsetRange(settings);
 	const { offsetX } = settings;
-	if (Math.abs(offsetX) < CAPTION_POSITION_PRESET_EPSILON) return "center";
-	if (Math.abs(offsetX - range.x.min) < CAPTION_POSITION_PRESET_EPSILON) return "left";
-	if (Math.abs(offsetX - range.x.max) < CAPTION_POSITION_PRESET_EPSILON) return "right";
-	return null;
+	const candidates: ReadonlyArray<[CaptionHorizontalPosition, number]> = [
+		["center", 0],
+		["left", range.x.min],
+		["right", range.x.max],
+	];
+	let closest: CaptionHorizontalPosition | null = null;
+	let closestDistance = CAPTION_POSITION_PRESET_EPSILON;
+	for (const [preset, target] of candidates) {
+		const distance = Math.abs(offsetX - target);
+		if (distance < closestDistance) {
+			closest = preset;
+			closestDistance = distance;
+		}
+	}
+	return closest;
 }
 
 /** The `offsetX` that puts the band at a given horizontal preset, for a preset