fix(base): prevent body from being scrolled by cross-frame scrollIntoView

Chrome's smooth cross-frame scrollIntoView walks up through frame boundaries
and lands on the parent's nearest scroll container. With `overflow: hidden`,
body is still treated as a scroll container that can be programmatically
scrolled, so when iframe content (e.g. the custom panel chat) auto-scrolls
on new content, the engine slides the entire body up by the off-screen
toolbar's height. Switching to `overflow: clip` removes body from the scroll
container chain entirely so the propagation has nowhere to land.

`overflow: hidden` is kept as a fallback for browsers that do not understand
`clip` (Chrome <90, Safari <16).

Author: hristoterezov

css/_base.scss

@@ -22,6 +22,7 @@ html {
     height: 100%;
     width: 100%;
     overflow: hidden;
+    overflow: clip;
 }
 
 body {
@@ -30,7 +31,14 @@ body {
     height: 100%;
     font-size: 0.75rem;
     font-weight: 400;
+    // `clip` (vs. `hidden`) makes the element a non-scroll container so the
+    // engine cannot programmatically scroll it. Prevents Chrome's smooth
+    // cross-frame `scrollIntoView` from sliding `body` up by the off-screen
+    // toolbar's height when content inside an iframe (e.g. the custom panel)
+    // requests an auto-scroll. The `hidden` line is the fallback for browsers
+    // that do not understand `clip` (Chrome <90, Safari <16).
     overflow: hidden;
+    overflow: clip;
     color: #F1F1F1;
     background: #040404; // should match DEFAULT_BACKGROUND from interface_config
 }