PY-89751 Restore cell rendering after collapse/expand of markdown headers

When a markdown header section is expanded, the previously hidden cell views are
recreated but only their folding is set up (createCellView), so their outputs and
inlays stay uninitialized until an unrelated event such as a scroll triggers
NotebookVisibleCellsBatchUpdater. This is a regression from the lazy output
initialization introduced in KTNB-812.

Refresh a cell view right after it becomes visible again in updateCellVisibility
(update + checkAndRebuildInlays + updateIfInVisibleRect) when the cell is inside the
viewport, so an expanded section renders immediately. Cells outside the viewport keep
being initialized lazily.

The previous attempt added an isUnfolded listener on EditorCell that called
updateIfInVisibleRect, but the cell view is always null at that point (it is disposed
while folded and only recreated by NotebookCellInlayManager afterwards), so it was a
no-op; it is removed.

Add a deterministic regression test (JupyterMarkdownFoldingRenderingTest) that inspects
the model right after the expand, and an end-to-end UI test in
JupyterPolygonIPyKernelUiTest.

M-Session-Id: M-b082df5b-7e57-46f6-aa59-496eef7d6cbe

GitOrigin-RevId: ef66dc9979025f9d4f0df472f96e8c032465a3e5

Author: nikita-ashihmin

notebooks/visualization/src/com/intellij/notebooks/visualization/NotebookCellInlayManager.kt

@@ -14,6 +14,7 @@ import com.intellij.notebooks.visualization.ui.EditorCellViewEventListener.CellV
 import com.intellij.notebooks.visualization.ui.EditorEmbeddedComponentContainer
 import com.intellij.notebooks.visualization.ui.EditorNotebook
 import com.intellij.notebooks.visualization.ui.JupyterCellSelectionNotifier
+import com.intellij.notebooks.visualization.ui.NotebookVisibleCellsBatchUpdater
 import com.intellij.notebooks.visualization.ui.endInlay.EditorNotebookEndInlay
 import com.intellij.notebooks.visualization.ui.endInlay.EditorNotebookEndInlayProvider
 import com.intellij.notebooks.visualization.ui.notebookViewUpdater
@@ -212,7 +213,18 @@ class NotebookCellInlayManager private constructor(
 
   private fun updateCellVisibility(cell: EditorCell, visible: Boolean) = update { ctx ->
     if (visible) {
+      val viewCreated = views[cell] == null
       createCellViewIfNecessary(cell, ctx)
+      // A cell becomes visible again after its enclosing markdown section is expanded (PY-89751).
+      // createCellView only sets up folding for the freshly recreated view, so its outputs and inlays stay
+      // uninitialized until an unrelated event (such as a scroll) triggers NotebookVisibleCellsBatchUpdater.
+      // Refresh the view right away when the cell is inside the viewport so the expanded section renders
+      // immediately; cells outside the viewport keep being initialized lazily (KTNB-812).
+      if (viewCreated && initialized.get() && NotebookVisibleCellsBatchUpdater.get(editor)?.isCellVisible(cell) == true) {
+        cell.update(ctx)
+        cell.checkAndRebuildInlays()
+        cell.updateIfInVisibleRect()
+      }
     }
     else {
       disposeCellView(cell)

notebooks/visualization/src/com/intellij/notebooks/visualization/ui/EditorCell.kt

@@ -39,12 +39,6 @@ class EditorCell(
     executionStatus.afterDistinctChange(this) {
       thisLogger().debug("Execution status changed: $it for interval ${intervalPointer.get()?.ordinal}")
     }
-
-    isUnfolded.afterDistinctChange(this) {
-      if (isUnfolded.get()) {
-        updateIfInVisibleRect()
-      }
-    }
   }
 
   val cellFrameManager: EditorCellFrameManager? = EditorCellFrameManager.create(this)?.also {