fix: Prevent content scrolling behind main menu (part 1) (#35173)

app/javascript/styles/mastodon/basics.scss

@@ -68,6 +68,7 @@ body {
     &.with-modals--active {
       overflow-y: hidden;
       overscroll-behavior: none;
+      margin-right: var(--root-scrollbar-width, 0);
     }
   }
 

app/javascript/styles/mastodon/components.scss

@@ -2894,6 +2894,11 @@ a.account__display-name {
   background: var(--background-color);
   backdrop-filter: var(--background-filter);
   border-top: 1px solid var(--background-border-color);
+  box-sizing: border-box;
+
+  .with-modals--active & {
+    padding-right: var(--root-scrollbar-width);
+  }
 
   .layout-multiple-columns & {
     display: none;