Include FromBorderSize & Scrollbars in Frame width. To fit responsive sizes properly.

layouts/scripts/toolbar/responsive.jsx

@@ -63,8 +63,13 @@ function Responsive(props = {}) {
   }
 
   function updateController() {
-    const unit = typeof breakpoint === 'string' ? '' : 'px';
-    previewFrame.style.setProperty('--breakpoint', breakpoint + unit);
+    let frameBreakpoint = breakpoint;
+    if (typeof frameBreakpoint !== 'string') {
+      const browserGap = 17; // Scrollbars + Borders
+      frameBreakpoint = (frameBreakpoint + browserGap) + 'px';
+    }
+
+    previewFrame.style.setProperty('--breakpoint', frameBreakpoint);
     previewFrame.classList.add('has-breakpoint');
     window.postMessage('responsiveUpdate');
     window.responsiveState = {mode, breakpoint};