grid keep sizing after branch node demotion

fixes #50564

src/vs/base/browser/ui/grid/gridview.ts

@@ -466,13 +466,21 @@ export class GridView implements IDisposable {
 
 		const sibling = parent.children[0];
 		parent.removeChild(0);
+
+		const sizes = grandParent.children.map(c => c.size);
 		grandParent.removeChild(parentIndex, sizing);
 
 		if (sibling instanceof BranchNode) {
+			sizes.splice(parentIndex, 1, ...sibling.children.map(c => c.size));
+
 			for (let i = 0; i < sibling.children.length; i++) {
 				const child = sibling.children[i];
 				grandParent.addChild(child, child.size, parentIndex + i);
 			}
+
+			for (let i = 0; i < sizes.length; i++) {
+				grandParent.resizeChild(i, sizes[i]);
+			}
 		} else {
 			const newSibling = new LeafNode(sibling.view, orthogonal(sibling.orientation), sibling.size);
 			grandParent.addChild(newSibling, sibling.orthogonalSize, parentIndex);

src/vs/base/test/browser/ui/grid/grid.test.ts

@@ -240,6 +240,30 @@ suite('Grid', function () {
 		assert.deepEqual(view4.size, [400, 300]);
 		assert.deepEqual(view5.size, [200, 300]);
 	});
+
+	test('sizing should be correct after branch demotion #50564', function () {
+		const view1 = new TestView(50, Number.MAX_VALUE, 50, Number.MAX_VALUE);
+		const grid = new Grid(container, view1);
+		grid.layout(800, 600);
+
+		const view2 = new TestView(50, Number.MAX_VALUE, 50, Number.MAX_VALUE);
+		grid.addView(view2, Sizing.Split, view1, Direction.Right);
+
+		const view3 = new TestView(50, Number.MAX_VALUE, 50, Number.MAX_VALUE);
+		grid.addView(view3, Sizing.Split, view2, Direction.Down);
+
+		const view4 = new TestView(50, Number.MAX_VALUE, 50, Number.MAX_VALUE);
+		grid.addView(view4, Sizing.Split, view2, Direction.Right);
+		assert.deepEqual(view1.size, [400, 600]);
+		assert.deepEqual(view2.size, [200, 300]);
+		assert.deepEqual(view3.size, [400, 300]);
+		assert.deepEqual(view4.size, [200, 300]);
+
+		grid.removeView(view3);
+		assert.deepEqual(view1.size, [400, 600]);
+		assert.deepEqual(view2.size, [200, 600]);
+		assert.deepEqual(view4.size, [200, 600]);
+	});
 });
 
 class TestSerializableView extends TestView implements ISerializableView {