Fix occasional scrollbars

Summary:
Sometimes after a while scroll bars woudl start to appear in the text areas like so
{F1150744703}

Its hard to repro but seemed to be after flipper was open for a long period. Playing with chrome dev tools setting overflow: hidden makes the problem go away

changelog: UIDebugger fix issue with scrollbars sometimes appearing in sidebar

Reviewed By: antonk52

Differential Revision: D51346366

fbshipit-source-id: 2d9218ac582164c9726a92c3c0c99567382f8929

desktop/plugins/public/ui-debugger/components/sidebarV2/AttributesInspector.tsx

@@ -315,6 +315,7 @@ function NamedAttribute({
  * disables hover and focsued states
  */
 const readOnlyInput = css`
+  overflow: hidden; //stop random scrollbars from showing up
   font-size: small;
   :hover {
     border-color: ${theme.disabledColor} !important;
@@ -388,7 +389,7 @@ function StyledTextArea({
   return (
     <Input.TextArea
       autoSize
-      className={!mutable ? readOnlyInput : ''}
+      className={cx(!mutable && readOnlyInput)}
       bordered
       style={{color: color}}
       readOnly={!mutable}