fix(masthead): prevent extra space beneath megapanel columns in Safari (

#11574)

### Related Ticket(s)

none

### Description

Fixes an issue that only presents on Safari where megamenu tabpanels are unnecessarily tall.

**Before:**
<img width="1841" alt="Screenshot 2024-02-22 at 3 44 56 PM" src="https://github.com/carbon-design-system/carbon-for-ibm-dotcom/assets/12532727/2648a3d6-f3b0-44b8-b6a1-970206b8ed0d">

**After:**
<img width="1841" alt="Screenshot 2024-02-22 at 3 44 47 PM" src="https://github.com/carbon-design-system/carbon-for-ibm-dotcom/assets/12532727/6520e5f5-6bf9-47eb-906d-285478a0ecd8">


### Changelog

**Changed**

- Prevents masthead megamenu tabpanels from displaying with excessive vertical space in Safari.


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packages/styles/scss/components/masthead/_masthead-megamenu.scss

@@ -125,10 +125,10 @@
     --#{$c4d-prefix}-masthead-max-height: calc(
       100vh - var(--cds-spacing-09, 3rem) - var(--cds-layout-05, 4rem)
     );
-    --#{$c4d-prefix}-masthead-overflow: overlay;
+    --#{$c4d-prefix}-masthead-overflow: auto;
 
     display: block;
-    overflow: hidden overlay;
+    overflow: hidden auto;
     background-color: $background;
 
     &:focus {
@@ -421,6 +421,16 @@
     display: block;
 
     &.only-child {
+      /**
+       * Necessary to accommodate Safari issue with columns.
+       *
+       * Container height is calculated before breaking into specified number
+       * of columns, which causes extra blank whitespace beneath column content.
+       */
+      .#{$prefix}--masthead__megamenu__category-group-content-wrapper {
+        display: inline;
+      }
+
       .#{$prefix}--masthead__megamenu__category-group-heading {
         @include make-col(1, 2);