diff --git a/src/display/canvas.js b/src/display/canvas.js
index 2781c19294b64..e104c9fbab920 100644
--- a/src/display/canvas.js
+++ b/src/display/canvas.js
@@ -1850,17 +1850,13 @@ class CanvasGraphics {
       return;
     }
 
-    // Whatever was drawn has been moved to the suspended canvas, now clear it
-    // out of the current canvas. Only the dirty box region needs clearing;
-    // everything outside it is already transparent.
+    // Clear the full scratch canvas, not just the dirty box. Pixels left
+    // outside dirtyBox can leak into a later compose() whose destination-in
+    // pass doesn't overwrite them, producing stale output -- this is what
+    // breaks `firefox-issue17779-partial` (issue #21276).
     this.ctx.save();
     this.ctx.setTransform(1, 0, 0, 1, 0, 0);
-    this.ctx.clearRect(
-      dirtyBox[0],
-      dirtyBox[1],
-      dirtyBox[2] - dirtyBox[0],
-      dirtyBox[3] - dirtyBox[1]
-    );
+    this.ctx.clearRect(0, 0, this.ctx.canvas.width, this.ctx.canvas.height);
     this.ctx.restore();
   }
 
diff --git a/src/display/canvas_dependency_tracker.js b/src/display/canvas_dependency_tracker.js
index ad26428b18bd7..11b6a564f6505 100644
--- a/src/display/canvas_dependency_tracker.js
+++ b/src/display/canvas_dependency_tracker.js
@@ -14,6 +14,7 @@
  */
 
 import { BBOX_INIT, FeatureTest, Util } from "../shared/util.js";
+import { getCurrentTransform } from "./display_utils.js";
 import { MathClamp } from "../shared/math_clamp.js";
 
 const FORCED_DEPENDENCY_LABEL = "__forcedDependency";
@@ -1121,7 +1122,7 @@ class CanvasImagesTracker {
       this.#coords = newCoords;
     }
 
-    const transform = Util.domMatrixToTransform(ctx.getTransform());
+    const transform = getCurrentTransform(ctx);
 
     // We want top left, bottom left, top right.
     // (0, 0) is the bottom left corner.
diff --git a/src/shared/util.js b/src/shared/util.js
index b75301b24c7b0..f73f6fc816b96 100644
--- a/src/shared/util.js
+++ b/src/shared/util.js
@@ -709,10 +709,6 @@ class Util {
     return `#${this.hexNums[r]}${this.hexNums[g]}${this.hexNums[b]}`;
   }
 
-  static domMatrixToTransform(dm) {
-    return [dm.a, dm.b, dm.c, dm.d, dm.e, dm.f];
-  }
-
   // Apply a scaling matrix to some min/max values.
   // If a scaling factor is negative then min and max must be
   // swapped.