feat(v2 Containers): scrollable containers (#6629)

apps/nextjs/src/components/board/sections/grid/section-grid.module.css

@@ -3,6 +3,28 @@
   overflow: visible;
 }
 
+/* Chained with .viewport (rather than a standalone .scrollableViewport rule) so this always
+   wins on specificity, not source order. GridEditor loads as a separate async chunk in edit
+   mode, and a board mutation can trigger its stylesheet to re-register - at equal specificity,
+   that flips which of these two rules the cascade picks, silently un-capping the container even
+   though its height (set inline, see section-grid.tsx) stays correct throughout. */
+.viewport.scrollableViewport {
+  overflow-x: hidden;
+  overflow-y: auto;
+  /* --board-grid-drag-height inherits down the DOM tree like any custom property. A capped
+     section's own GridPreviewLayer effect correctly avoids setting it on itself (see
+     grid-preview-layer.tsx), but that does nothing to stop it inheriting the value an
+     *uncapped* ancestor section (e.g. the board's root section) legitimately sets on itself
+     while growing during a drag happening anywhere else on the board - a nested scrollable
+     container sits inside that section's DOM tree, so the stray value cascades straight
+     through and silently defeats the `var(--board-grid-drag-height, ...px)` fallback in
+     section-grid.tsx, which only kicks in when the property is entirely unset, not merely
+     inherited. Reset it here so this element can never inherit a stray value from anywhere
+     above it, no matter what any ancestor's own drag state is doing.
+     https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascading_variables#custom_properties_and_inheritance */
+  --board-grid-drag-height: initial;
+}
+
 .staticGrid,
 .editorGrid {
   position: relative;