Placing the textarea inside of a container is the way to go. If you’re only targeting one device, then go for absolute positioning (a property on containers and some controls). If you want to support many or all devices, you’ll want to look at responsive in the discussion board and in particular this post https://discuss.appstudio.dev/t/app-design-guide-trends-rules/1785/3.
I don’t worry about how items look in the ide. (in windows the controls are really screwed up in the IDE). And if you go responsive, you may find not using the grid approach may fit your needs better. Use inline-grid and/or flex where you want stuff to flow with screen size and add lots of container levels to get the effects you want. But you may not realize up front what that is.
And my last suggestion is to forget about what you think the screen should look like. What? Let the browser flow things and then adjust that flow to get the desired effect. I like the ability to slide the width of the view port on chrome and watch how the flow works as the size changes. (turning on the inspector makes sliding the viewport width easy). The IDE is not WYSIWYG.