Fix nested non-isolated groups in knockout rendering

It fixes #18032.

Only use the special inner-backdrop compositing path for nested non-isolated groups that actually need isolation.
This preserves the parent/page backdrop for simple nested groups inside knockout groups, preventing later group
compositing from erasing existing backdrop content.

src/display/canvas.js

@@ -3342,13 +3342,15 @@ class CanvasGraphics {
     // #compositeKnockoutSurface so it doesn't become part of the group
     // source itself.
     const backdropCtx = group.knockout && !group.isolated ? currentCtx : null;
-    // Non-isolated non-KO subgroup inside a KO parent: at endGroup we'll
-    // blend its elements against the outer KO running canvas (also frozen),
-    // so just record the flag here and read ctx.canvas at composite time.
+    // Non-isolated non-KO subgroup inside a KO parent, with inner compositing
+    // of its own: at endGroup we'll blend its elements against the outer KO
+    // running canvas (also frozen), so just record the flag here and read
+    // ctx.canvas at composite time.
     const hasInnerBackdrop =
       !group.isolated &&
       !group.knockout &&
       !group.smask &&
+      group.needsIsolation &&
       this.#knockoutGroupLevel > 0;
 
     // Pool the per-element shape mask for the lifetime of this KO group.

test/pdfs/.gitignore

@@ -920,3 +920,4 @@
 !smask_luminosity_oob_transfer.pdf
 !operator_list_cycle.pdf
 !knockout_groups_test.pdf
+!issue18032.pdf

test/test_manifest.json

@@ -14252,5 +14252,13 @@
     "rounds": 1,
     "type": "eq",
     "about": "Knockout groups composite-modes survey."
+  },
+  {
+    "id": "issue18032",
+    "file": "pdfs/issue18032.pdf",
+    "md5": "656234fde97953c4e3fc7698831590a9",
+    "rounds": 1,
+    "type": "eq",
+    "about": "Non-isolated knockout group with a nested non-isolated subgroup over existing text."
   }
 ]