diff --git a/src/core/animations/effect-preset-builder.ts b/src/core/animations/effect-preset-builder.ts
index cf45ebb7..ff367d85 100644
--- a/src/core/animations/effect-preset-builder.ts
+++ b/src/core/animations/effect-preset-builder.ts
@@ -59,17 +59,19 @@ export class EffectPresetBuilder {
 			case "slideUp":
 			case "slideDown": {
 				const isHorizontal = effectName === "slideLeft" || effectName === "slideRight";
-				const startsPositive = effectName === "slideLeft" || effectName === "slideUp";
+				// Left and down start on the positive side of their axis (offset y points up).
+				const startsPositive = effectName === "slideLeft" || effectName === "slideDown";
 
-				const fittedSize = this.getFittedSize(editSize, clipSize);
+				// The render checks for overflow with the media fitted to the frame's other side, whatever the clip's fit.
 				const editDimension = isHorizontal ? editSize.width : editSize.height;
-				const fittedDimension = isHorizontal ? fittedSize.width : fittedSize.height;
+				const fittedDimension = isHorizontal
+					? (clipSize.width / clipSize.height) * editSize.height
+					: (clipSize.height / clipSize.width) * editSize.width;
 
 				let targetOffset = this.getSlideStart();
-				const minScale = editDimension + editDimension * targetOffset * 2;
+				const scaleFactor = 1 + targetOffset * 2;
 
-				if (fittedDimension < minScale) {
-					const scaleFactor = minScale / fittedDimension;
+				if (fittedDimension < editDimension * scaleFactor) {
 					scaleKeyframes.push({ from: scaleFactor, to: scaleFactor, start, length, interpolation: "linear" });
 				} else {
 					targetOffset = (fittedDimension - editDimension) / 2 / editDimension;
@@ -111,36 +113,18 @@ export class EffectPresetBuilder {
 	private getSlideStart(): number {
 		const { name, speed } = this.effectPreset;
 
+		// Fractions of the frame, matching the render.
 		if (name.startsWith("slide")) {
 			switch (speed) {
 				case "Slow":
 					return 0.03;
 				case "Fast":
-					return 0.2;
-				default:
 					return 0.12;
+				default:
+					return 0.08;
 			}
 		}
 
 		return 0;
 	}
-
-	private getFittedSize(editSize: Size, clipSize: Size): Size {
-		const fit = this.clipConfiguration.fit ?? "crop";
-
-		switch (fit) {
-			case "cover":
-			case "crop": {
-				const scale = Math.max(editSize.width / clipSize.width, editSize.height / clipSize.height);
-				return { width: clipSize.width * scale, height: clipSize.height * scale };
-			}
-			case "contain": {
-				const scale = Math.min(editSize.width / clipSize.width, editSize.height / clipSize.height);
-				return { width: clipSize.width * scale, height: clipSize.height * scale };
-			}
-			case "none":
-			default:
-				return clipSize;
-		}
-	}
 }
diff --git a/tests/effect-preset-builder.test.ts b/tests/effect-preset-builder.test.ts
new file mode 100644
index 00000000..75f1b100
--- /dev/null
+++ b/tests/effect-preset-builder.test.ts
@@ -0,0 +1,48 @@
+import { EffectPresetBuilder } from "@animations/effect-preset-builder";
+import { sec } from "@core/timing/types";
+import type { ResolvedClip } from "@schemas";
+
+const EDIT = { width: 1920, height: 1080 };
+
+const build = (effect: string, clipSize = EDIT, fit: ResolvedClip["fit"] = "crop") =>
+	new EffectPresetBuilder({
+		id: "clip-1",
+		asset: { type: "image", src: "https://example.com/a.png" },
+		start: sec(0),
+		length: sec(5),
+		fit,
+		effect: effect as ResolvedClip["effect"]
+	}).buildRelative(EDIT, clipSize);
+
+describe("EffectPresetBuilder", () => {
+	it.each([
+		["slideLeft", 0.08],
+		["slideLeftSlow", 0.03],
+		["slideLeftFast", 0.12]
+	])("pans %s by the render's %s of the frame, enlarged to cover it", (effect, distance) => {
+		const { offsetXKeyframes, scaleKeyframes } = build(effect);
+
+		expect(offsetXKeyframes).toEqual([{ from: distance, to: -distance, start: 0, length: 5 }]);
+		expect(scaleKeyframes[0]).toMatchObject({ from: 1 + 2 * distance, to: 1 + 2 * distance });
+	});
+
+	it.each([
+		["slideUp", -0.08, 0.08],
+		["slideDown", 0.08, -0.08]
+	])("pans %s the way its name says (offset y points up)", (effect, from, to) => {
+		expect(build(effect).offsetYKeyframes).toEqual([{ from, to, start: 0, length: 5 }]);
+	});
+
+	it("enlarges narrow media by the same amount whatever its fit", () => {
+		const { scaleKeyframes } = build("slideLeft", { width: 1080, height: 1920 }, "contain");
+
+		expect(scaleKeyframes[0]).toMatchObject({ from: 1.16, to: 1.16 });
+	});
+
+	it.each(["crop", "contain"] as const)("pans the overflow of media fitted to the frame's height (%s)", fit => {
+		const { offsetXKeyframes, scaleKeyframes } = build("slideRight", { width: 4000, height: 1000 }, fit);
+
+		expect(offsetXKeyframes).toEqual([{ from: -0.625, to: 0.625, start: 0, length: 5 }]);
+		expect(scaleKeyframes).toEqual([]);
+	});
+});