diff --git a/src/lib/layout-presets.test.ts b/src/lib/layout-presets.test.ts
new file mode 100644
index 0000000..beacdd2
--- /dev/null
+++ b/src/lib/layout-presets.test.ts
@@ -0,0 +1,261 @@
+import { describe, it, expect } from "vitest";
+import {
+  collectWindowIds,
+  applyPresetToLayout,
+  BUILT_IN_PRESETS,
+} from "./layout-presets";
+import type { MosaicNode } from "react-mosaic-component";
+
+describe("layout-presets", () => {
+  describe("collectWindowIds", () => {
+    it("collects IDs from single window", () => {
+      expect(collectWindowIds("w1")).toEqual(["w1"]);
+    });
+
+    it("collects IDs from binary tree", () => {
+      const layout: MosaicNode<string> = {
+        direction: "row",
+        first: "w1",
+        second: "w2",
+        splitPercentage: 50,
+      };
+      expect(collectWindowIds(layout)).toEqual(["w1", "w2"]);
+    });
+
+    it("collects IDs in depth-first order", () => {
+      const layout: MosaicNode<string> = {
+        direction: "row",
+        first: {
+          direction: "column",
+          first: "w1",
+          second: "w2",
+          splitPercentage: 50,
+        },
+        second: {
+          direction: "column",
+          first: "w3",
+          second: "w4",
+          splitPercentage: 50,
+        },
+        splitPercentage: 50,
+      };
+      expect(collectWindowIds(layout)).toEqual(["w1", "w2", "w3", "w4"]);
+    });
+  });
+
+  describe("grid preset", () => {
+    const gridPreset = BUILT_IN_PRESETS.grid;
+
+    it("handles 2 windows (1x2 grid)", () => {
+      const layout = gridPreset.generate(["w1", "w2"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2"]);
+      expect(windowIds.length).toBe(2);
+    });
+
+    it("handles 3 windows (1x3 single row)", () => {
+      const layout = gridPreset.generate(["w1", "w2", "w3"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3"]);
+      expect(windowIds.length).toBe(3);
+    });
+
+    it("handles 4 windows (2x2 perfect grid)", () => {
+      const layout = gridPreset.generate(["w1", "w2", "w3", "w4"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3", "w4"]);
+      expect(windowIds.length).toBe(4);
+    });
+
+    it("handles 5 windows (2x3 with expanded last row)", () => {
+      const layout = gridPreset.generate(["w1", "w2", "w3", "w4", "w5"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3", "w4", "w5"]);
+      expect(windowIds.length).toBe(5);
+      // Should create 2 rows: [w1,w2,w3] and [w4,w5]
+      // Last row windows expand to fill space (no empty slots)
+    });
+
+    it("handles 7 windows (2x4 with expanded last row)", () => {
+      const layout = gridPreset.generate([
+        "w1",
+        "w2",
+        "w3",
+        "w4",
+        "w5",
+        "w6",
+        "w7",
+      ]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3", "w4", "w5", "w6", "w7"]);
+      expect(windowIds.length).toBe(7);
+      // Should create 2 rows: [w1,w2,w3,w4] and [w5,w6,w7]
+    });
+
+    it("handles 9 windows (3x3 perfect grid)", () => {
+      const layout = gridPreset.generate([
+        "w1",
+        "w2",
+        "w3",
+        "w4",
+        "w5",
+        "w6",
+        "w7",
+        "w8",
+        "w9",
+      ]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds.length).toBe(9);
+    });
+
+    it("handles 11 windows (3x4 with expanded last row)", () => {
+      const layout = gridPreset.generate([
+        "w1",
+        "w2",
+        "w3",
+        "w4",
+        "w5",
+        "w6",
+        "w7",
+        "w8",
+        "w9",
+        "w10",
+        "w11",
+      ]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds.length).toBe(11);
+      // Should create 3 rows: [w1-4], [w5-8], [w9-11]
+    });
+
+    it("preserves window order in depth-first traversal", () => {
+      const original = ["a", "b", "c", "d", "e"];
+      const layout = gridPreset.generate(original);
+      const collected = collectWindowIds(layout);
+      expect(collected).toEqual(original);
+    });
+  });
+
+  describe("side-by-side preset", () => {
+    const sideBySidePreset = BUILT_IN_PRESETS["side-by-side"];
+
+    it("handles 2 windows (50/50)", () => {
+      const layout = sideBySidePreset.generate(["w1", "w2"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2"]);
+    });
+
+    it("handles 3 windows (equal splits)", () => {
+      const layout = sideBySidePreset.generate(["w1", "w2", "w3"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3"]);
+    });
+
+    it("handles 4 windows (max allowed)", () => {
+      const layout = sideBySidePreset.generate(["w1", "w2", "w3", "w4"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3", "w4"]);
+    });
+
+    it("throws error for 5+ windows", () => {
+      expect(() =>
+        sideBySidePreset.generate(["w1", "w2", "w3", "w4", "w5"])
+      ).toThrow("maximum 4 windows");
+    });
+  });
+
+  describe("main-sidebar preset", () => {
+    const mainSidebarPreset = BUILT_IN_PRESETS["main-sidebar"];
+
+    it("handles 2 windows (main + 1 sidebar)", () => {
+      const layout = mainSidebarPreset.generate(["w1", "w2"]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2"]);
+    });
+
+    it("handles 5 windows (main + 4 sidebars)", () => {
+      const layout = mainSidebarPreset.generate([
+        "w1",
+        "w2",
+        "w3",
+        "w4",
+        "w5",
+      ]);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(["w1", "w2", "w3", "w4", "w5"]);
+      // First window is main, rest are stacked vertically
+    });
+
+    it("handles 10 windows (main + 9 sidebars)", () => {
+      const windows = Array.from({ length: 10 }, (_, i) => `w${i + 1}`);
+      const layout = mainSidebarPreset.generate(windows);
+      const windowIds = collectWindowIds(layout);
+      expect(windowIds).toEqual(windows);
+    });
+  });
+
+  describe("applyPresetToLayout", () => {
+    it("throws error if too few windows", () => {
+      const layout: MosaicNode<string> = "w1";
+      expect(() =>
+        applyPresetToLayout(layout, BUILT_IN_PRESETS.grid)
+      ).toThrow("at least 2 windows");
+    });
+
+    it("throws error if too many windows for side-by-side", () => {
+      const layout: MosaicNode<string> = {
+        direction: "row",
+        first: {
+          direction: "row",
+          first: "w1",
+          second: "w2",
+          splitPercentage: 50,
+        },
+        second: {
+          direction: "row",
+          first: {
+            direction: "row",
+            first: "w3",
+            second: "w4",
+            splitPercentage: 50,
+          },
+          second: "w5",
+          splitPercentage: 50,
+        },
+        splitPercentage: 50,
+      };
+      expect(() =>
+        applyPresetToLayout(layout, BUILT_IN_PRESETS["side-by-side"])
+      ).toThrow("maximum 4 windows");
+    });
+
+    it("applies grid preset to existing layout", () => {
+      const existingLayout: MosaicNode<string> = {
+        direction: "row",
+        first: "w1",
+        second: "w2",
+        splitPercentage: 50,
+      };
+      const result = applyPresetToLayout(existingLayout, BUILT_IN_PRESETS.grid);
+      const windowIds = collectWindowIds(result);
+      expect(windowIds).toEqual(["w1", "w2"]);
+    });
+
+    it("preserves all windows when applying preset", () => {
+      const originalWindows = ["w1", "w2", "w3", "w4", "w5", "w6", "w7"];
+      // Create a simple layout
+      let layout: MosaicNode<string> = originalWindows[0];
+      for (let i = 1; i < originalWindows.length; i++) {
+        layout = {
+          direction: "row",
+          first: layout,
+          second: originalWindows[i],
+          splitPercentage: 50,
+        };
+      }
+
+      const result = applyPresetToLayout(layout, BUILT_IN_PRESETS.grid);
+      const windowIds = collectWindowIds(result);
+      expect(windowIds).toEqual(originalWindows);
+    });
+  });
+});