fix(board): reserve and scale space for collapsible container header (#6727)

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

@@ -3,6 +3,22 @@
   overflow: visible;
 }
 
+/* --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 - any container (not just
+   a scrollable one - a non-scrollable, e.g. collapsible, container is just as capped) 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 (including
+   its collapsibleHeaderInset adjustment), which only kicks in when the property is entirely
+   unset, not merely inherited. Reset it here so every container 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 */
+.viewport[data-section-kind="container"] {
+  --board-grid-drag-height: initial;
+}
+
 /* 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,
@@ -11,17 +27,14 @@
 .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 */
+  /* The grid content inside has a fixed pixel width matching this viewport's full
+     outer width (needed for the drag/resize math), so there's no room to reserve
+     for a scrollbar without it eating into - or overlapping - the content on
+     whichever side it renders. Hiding the native scrollbar track entirely (wheel/
+     touch/drag scrolling still works) avoids that instead of trying to reserve
+     space for it. */
+  scrollbar-width: none;
+  -ms-overflow-style: none;
   --board-grid-drag-height: initial;
 }