src/vs/workbench/contrib/comments/browser/commentThreadWidget.ts/ReviewZoneWidget/setCommentEditorDecorations
class ReviewZoneWidget:	private setCommentEditorDecorations() {
		const model = this._commentReplyComponent && this._commentReplyComponent.editor.getModel();
		if (model) {
			const valueLength = model.getValueLength();
			const hasExistingComments = this._commentThread.comments && this._commentThread.comments.length > 0;
			const placeholder = valueLength > 0
				? ''
				: hasExistingComments
					? (this._commentOptions?.placeHolder || nls.localize('reply', "Reply..."))
					: (this._commentOptions?.placeHolder || nls.localize('newComment', "Type a new comment"));
			const decorations = [{
				range: {
					startLineNumber: 0,
					endLineNumber: 0,
					startColumn: 0,
					endColumn: 1
				},
				renderOptions: {
					after: {
						contentText: placeholder,
						color: `${resolveColorValue(editorForeground, this.themeService.getColorTheme())?.transparent(0.4)}`
					}
				}
			}];

			this._commentReplyComponent?.editor.setDecorations('review-zone-widget', COMMENTEDITOR_DECORATION_KEY, decorations);
		}
	}