diff --git a/packages/vue-split-panel/src/SplitPanel.vue b/packages/vue-split-panel/src/SplitPanel.vue
index b15560e..46d88d5 100644
--- a/packages/vue-split-panel/src/SplitPanel.vue
+++ b/packages/vue-split-panel/src/SplitPanel.vue
@@ -1,6 +1,7 @@
 <script lang="ts" setup>
 import type { SplitPanelProps } from './types';
-import { ref, useTemplateRef, watch } from 'vue';
+import { useTemplateRef } from 'vue';
+import { useCollapse } from './composables/use-collapse';
 import { useGridTemplate } from './composables/use-grid-template';
 import { useKeyboard } from './composables/use-keyboard';
 import { usePointer } from './composables/use-pointer';
@@ -35,10 +36,6 @@ const collapsed = defineModel<boolean>('collapsed', { default: false });
 const panelEl = useTemplateRef('split-panel');
 const dividerEl = useTemplateRef('divider');
 
-let expandedSizePercentage = 0;
-
-const collapseTransitionState = ref<null | 'expanding' | 'collapsing'>(null);
-
 const {
 	sizePercentage,
 	sizePixels,
@@ -103,26 +100,7 @@ useResize(sizePercentage, {
 	primary: () => props.primary,
 });
 
-const onTransitionEnd = (event: TransitionEvent) => {
-	collapseTransitionState.value = null;
-	emits('transitionend', event);
-};
-
-watch(collapsed, (newCollapsed) => {
-	if (newCollapsed === true) {
-		expandedSizePercentage = sizePercentage.value;
-		sizePercentage.value = 0;
-		collapseTransitionState.value = 'collapsing';
-	}
-	else {
-		sizePercentage.value = expandedSizePercentage;
-		collapseTransitionState.value = 'expanding';
-	}
-});
-
-const collapse = () => collapsed.value = true;
-const expand = () => collapsed.value = false;
-const toggle = (val: boolean) => collapsed.value = val;
+const { onTransitionEnd, collapseTransitionState, toggle, expand, collapse } = useCollapse(collapsed, sizePercentage, emits);
 
 defineExpose({ collapse, expand, toggle });
 </script>
diff --git a/packages/vue-split-panel/src/composables/use-collapse.test.ts b/packages/vue-split-panel/src/composables/use-collapse.test.ts
new file mode 100644
index 0000000..9c50f0a
--- /dev/null
+++ b/packages/vue-split-panel/src/composables/use-collapse.test.ts
@@ -0,0 +1,312 @@
+import { describe, expect, it, vi } from 'vitest';
+import { nextTick, ref } from 'vue';
+import { useCollapse } from './use-collapse';
+
+describe('useCollapse', () => {
+	it('should return expected methods and properties', () => {
+		const collapsed = ref(false);
+		const sizePercentage = ref(50);
+		const emits = vi.fn();
+
+		const result = useCollapse(collapsed, sizePercentage, emits);
+
+		expect(result).toHaveProperty('onTransitionEnd');
+		expect(result).toHaveProperty('collapse');
+		expect(result).toHaveProperty('expand');
+		expect(result).toHaveProperty('toggle');
+		expect(result).toHaveProperty('collapseTransitionState');
+		expect(typeof result.onTransitionEnd).toBe('function');
+		expect(typeof result.collapse).toBe('function');
+		expect(typeof result.expand).toBe('function');
+		expect(typeof result.toggle).toBe('function');
+		expect(result.collapseTransitionState.value).toBeNull();
+	});
+
+	describe('collapse method', () => {
+		it('should set collapsed to true', () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(50);
+			const emits = vi.fn();
+
+			const { collapse } = useCollapse(collapsed, sizePercentage, emits);
+
+			collapse();
+
+			expect(collapsed.value).toBe(true);
+		});
+	});
+
+	describe('expand method', () => {
+		it('should set collapsed to false', () => {
+			const collapsed = ref(true);
+			const sizePercentage = ref(0);
+			const emits = vi.fn();
+
+			const { expand } = useCollapse(collapsed, sizePercentage, emits);
+
+			expand();
+
+			expect(collapsed.value).toBe(false);
+		});
+	});
+
+	describe('toggle method', () => {
+		it('should set collapsed to the provided value', () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(50);
+			const emits = vi.fn();
+
+			const { toggle } = useCollapse(collapsed, sizePercentage, emits);
+
+			toggle(true);
+			expect(collapsed.value).toBe(true);
+
+			toggle(false);
+			expect(collapsed.value).toBe(false);
+		});
+	});
+
+	describe('collapsed watcher behavior', () => {
+		it('should store size and set to 0 when collapsing', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(75);
+			const emits = vi.fn();
+
+			const { collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			collapsed.value = true;
+			await nextTick();
+
+			expect(sizePercentage.value).toBe(0);
+			expect(collapseTransitionState.value).toBe('collapsing');
+		});
+
+		it('should restore size when expanding', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(60);
+			const emits = vi.fn();
+
+			const { collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			// First collapse to store the size
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			// Then expand to restore
+			collapsed.value = false;
+			await nextTick();
+
+			expect(sizePercentage.value).toBe(60);
+			expect(collapseTransitionState.value).toBe('expanding');
+		});
+
+		it('should preserve original size through multiple collapse/expand cycles', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(42);
+			const emits = vi.fn();
+
+			useCollapse(collapsed, sizePercentage, emits);
+
+			// First cycle
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			collapsed.value = false;
+			await nextTick();
+			expect(sizePercentage.value).toBe(42);
+
+			// Second cycle
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			collapsed.value = false;
+			await nextTick();
+			expect(sizePercentage.value).toBe(42);
+		});
+
+		it('should handle size changes between collapse cycles', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(30);
+			const emits = vi.fn();
+
+			useCollapse(collapsed, sizePercentage, emits);
+
+			// First collapse
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			// Expand and change size
+			collapsed.value = false;
+			await nextTick();
+			expect(sizePercentage.value).toBe(30);
+
+			// Manually change size while expanded
+			sizePercentage.value = 80;
+
+			// Collapse again - should store new size
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			// Expand - should restore new size
+			collapsed.value = false;
+			await nextTick();
+			expect(sizePercentage.value).toBe(80);
+		});
+	});
+
+	describe('transition state management', () => {
+		it('should start with null transition state', () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(50);
+			const emits = vi.fn();
+
+			const { collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			expect(collapseTransitionState.value).toBeNull();
+		});
+
+		it('should set collapsing state when collapsed becomes true', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(50);
+			const emits = vi.fn();
+
+			const { collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			collapsed.value = true;
+			await nextTick();
+
+			expect(collapseTransitionState.value).toBe('collapsing');
+		});
+
+		it('should set expanding state when collapsed becomes false', async () => {
+			const collapsed = ref(true);
+			const sizePercentage = ref(0);
+			const emits = vi.fn();
+
+			const { collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			collapsed.value = false;
+			await nextTick();
+
+			expect(collapseTransitionState.value).toBe('expanding');
+		});
+	});
+
+	describe('onTransitionEnd', () => {
+		it('should clear transition state and emit event', () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(50);
+			const emits = vi.fn();
+
+			const { onTransitionEnd, collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			// Set a transition state first
+			collapseTransitionState.value = 'collapsing';
+
+			const mockEvent = new TransitionEvent('transitionend', {
+				propertyName: 'grid-template-columns',
+				elapsedTime: 0.3,
+			});
+
+			onTransitionEnd(mockEvent);
+
+			expect(collapseTransitionState.value).toBeNull();
+			expect(emits).toHaveBeenCalledWith('transitionend', mockEvent);
+		});
+
+		it('should work with expanding state', () => {
+			const collapsed = ref(true);
+			const sizePercentage = ref(0);
+			const emits = vi.fn();
+
+			const { onTransitionEnd, collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			collapseTransitionState.value = 'expanding';
+
+			const mockEvent = new TransitionEvent('transitionend');
+			onTransitionEnd(mockEvent);
+
+			expect(collapseTransitionState.value).toBeNull();
+			expect(emits).toHaveBeenCalledWith('transitionend', mockEvent);
+		});
+	});
+
+	describe('integration scenarios', () => {
+		it('should handle rapid collapse/expand operations', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(65);
+			const emits = vi.fn();
+
+			const { collapseTransitionState, onTransitionEnd } = useCollapse(collapsed, sizePercentage, emits);
+
+			// Rapid collapse
+			collapsed.value = true;
+			await nextTick();
+			expect(collapseTransitionState.value).toBe('collapsing');
+			expect(sizePercentage.value).toBe(0);
+
+			// Immediate expand before transition ends
+			collapsed.value = false;
+			await nextTick();
+			expect(collapseTransitionState.value).toBe('expanding');
+			expect(sizePercentage.value).toBe(65);
+
+			// Simulate transition end
+			const mockEvent = new TransitionEvent('transitionend');
+			onTransitionEnd(mockEvent);
+			expect(collapseTransitionState.value).toBeNull();
+		});
+
+		it('should work with methods triggering state changes', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(45);
+			const emits = vi.fn();
+
+			const { collapse, expand, toggle, collapseTransitionState } = useCollapse(collapsed, sizePercentage, emits);
+
+			// Use collapse method
+			collapse();
+			await nextTick();
+			expect(collapsed.value).toBe(true);
+			expect(sizePercentage.value).toBe(0);
+			expect(collapseTransitionState.value).toBe('collapsing');
+
+			// Use expand method
+			expand();
+			await nextTick();
+			expect(collapsed.value).toBe(false);
+			expect(sizePercentage.value).toBe(45);
+			expect(collapseTransitionState.value).toBe('expanding');
+
+			// Use toggle method
+			toggle(true);
+			await nextTick();
+			expect(collapsed.value).toBe(true);
+			expect(sizePercentage.value).toBe(0);
+			expect(collapseTransitionState.value).toBe('collapsing');
+		});
+
+		it('should work with zero initial size', async () => {
+			const collapsed = ref(false);
+			const sizePercentage = ref(0);
+			const emits = vi.fn();
+
+			useCollapse(collapsed, sizePercentage, emits);
+
+			// Collapse when already at 0
+			collapsed.value = true;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+
+			// Expand - should restore to 0
+			collapsed.value = false;
+			await nextTick();
+			expect(sizePercentage.value).toBe(0);
+		});
+	});
+});
diff --git a/packages/vue-split-panel/src/composables/use-collapse.ts b/packages/vue-split-panel/src/composables/use-collapse.ts
new file mode 100644
index 0000000..f2d88f2
--- /dev/null
+++ b/packages/vue-split-panel/src/composables/use-collapse.ts
@@ -0,0 +1,31 @@
+import type { Ref } from 'vue';
+import { ref, watch } from 'vue';
+
+export const useCollapse = (collapsed: Ref<boolean>, sizePercentage: Ref<number>, emits: (evt: 'transitionend', event: TransitionEvent) => void) => {
+	let expandedSizePercentage = 0;
+
+	const collapseTransitionState = ref<null | 'expanding' | 'collapsing'>(null);
+
+	const onTransitionEnd = (event: TransitionEvent) => {
+		collapseTransitionState.value = null;
+		emits('transitionend', event);
+	};
+
+	watch(collapsed, (newCollapsed) => {
+		if (newCollapsed === true) {
+			expandedSizePercentage = sizePercentage.value;
+			sizePercentage.value = 0;
+			collapseTransitionState.value = 'collapsing';
+		}
+		else {
+			sizePercentage.value = expandedSizePercentage;
+			collapseTransitionState.value = 'expanding';
+		}
+	});
+
+	const collapse = () => collapsed.value = true;
+	const expand = () => collapsed.value = false;
+	const toggle = (val: boolean) => collapsed.value = val;
+
+	return { onTransitionEnd, collapse, expand, toggle, collapseTransitionState };
+};