[Layout component] Avoid resizing main while loading (#1818)

* avoid resizing `main` while loading

* Update layout.scss

* Create stale-pugs-eat.md

Co-authored-by: Jon Rohan <[email protected]>

Author: vdepizzol

.changeset/stale-pugs-eat.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+[Layout component] Avoid resizing `main` while loading

src/layout/layout.scss

@@ -25,7 +25,7 @@
   // Flow as column
 
   grid-auto-flow: column;
-  grid-template-columns: auto 0 1fr; // sidebar column, separator, main column
+  grid-template-columns: auto 0 minmax(0, calc(100% - var(--Layout-sidebar-width) - var(--Layout-gutter))); // sidebar column, separator, main column
   grid-gap: var(--Layout-gutter);
 
   .Layout-sidebar {
@@ -101,7 +101,7 @@
   }
 
   &.Layout--sidebarPosition-end {
-    grid-template-columns: 1fr 0 auto;
+    grid-template-columns: minmax(0, calc(100% - var(--Layout-sidebar-width) - var(--Layout-gutter))) 0 auto;
 
     .Layout-main {
       grid-column: 1;