UX: Composer and sidebar better fit (#23841)

app/assets/stylesheets/desktop/compose.scss

@@ -324,3 +324,37 @@ a.toggle-preview {
     background: none;
   }
 }
+
+@media screen and (min-width: $reply-area-max-width) {
+  .sidebar-wrapper .sidebar-container {
+    .has-sidebar-page & {
+      height: auto;
+    }
+  }
+  #reply-control:not(.fullscreen) {
+    .has-sidebar-page & {
+      width: auto;
+      max-width: calc(
+        (var(--d-max-width) + (var(--d-sidebar-width) / 2)) - 1.5rem
+      );
+      margin-left: 0;
+      margin-right: 0;
+      transform: translateX(
+        calc(
+          ((100vw - var(--d-max-width) - (var(--d-sidebar-width) / 0.5)) / 2) +
+            17em + 1rem
+        )
+      );
+
+      &.hide-preview {
+        max-width: calc(740px);
+        transform: translateX(
+          calc(
+            ((100vw - 740px - var(--d-sidebar-width)) / 2) +
+              var(--d-sidebar-width)
+          )
+        );
+      }
+    }
+  }
+}