diff --git a/src/vs/workbench/contrib/chat/browser/chatInputWindow/chatInputWindowService.ts b/src/vs/workbench/contrib/chat/browser/chatInputWindow/chatInputWindowService.ts
index e0749a05ff9..efceae4c8c5 100644
--- a/src/vs/workbench/contrib/chat/browser/chatInputWindow/chatInputWindowService.ts
+++ b/src/vs/workbench/contrib/chat/browser/chatInputWindow/chatInputWindowService.ts
@@ -642,7 +642,15 @@ export class ChatInputWindowService extends Disposable implements IChatInputWind
 		let pendingBounds: IRectangle | undefined;
 		let applyingBounds = false;
 		const getRowHeight = () => {
-			let contentHeight = Math.ceil(widget.contentHeight);
+			// Measure the input's rendered height synchronously rather than
+			// reading `widget.contentHeight`, which is backed by a
+			// ResizeObserver and lags a frame behind. `layout()` re-lays the
+			// editor synchronously, so the observable is stale until the
+			// observer fires next frame; sizing the window to that stale height
+			// while the editor grows (e.g. pasting text or cycling input
+			// history) clips the new lines and makes the window grow
+			// inconsistently until the observer catches up.
+			let contentHeight = Math.ceil(widget.input.element.offsetHeight);
 			if (widget.attachmentModel.size > 0) {
 				contentHeight += Math.max(0, CHAT_INPUT_WINDOW_INITIAL_SURFACE_HEIGHT - widget.input.inputRowHeight);
 			}