Overlay: Fix position for fullscreen overlay on narrow screens (#5835)

Co-authored-by: Marie Lucca <40550942+francinelucca@users.noreply.github.com>

Author: siddharthkp

.changeset/light-beds-stare.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": patch
+---
+
+Overlay: Fix position for fullscreen overlay on narrow screens

packages/react/src/Overlay/Overlay.module.css

@@ -158,6 +158,7 @@
   }
 
   &:where([data-variant='fullscreen']) {
+    position: fixed;
     top: 0;
     left: 0;
     width: 100vw;

packages/react/src/Overlay/Overlay.tsx

@@ -104,6 +104,7 @@ const StyledOverlay = toggleStyledComponent(
     }
 
     &:where([data-variant='fullscreen']) {
+      position: fixed;
       top: 0;
       left: 0;
       width: 100vw;

packages/react/src/__tests__/__snapshots__/AnchoredOverlay.test.tsx.snap

@@ -31,6 +31,7 @@ exports[`AnchoredOverlay should render consistently when open 1`] = `
 }
 
 .c1:where([data-variant='fullscreen']) {
+  position: fixed;
   top: 0;
   left: 0;
   width: 100vw;