[PDF Ink Signatures] Modify timing to drop Ink input snapshot

The change in https://crrev.com/1433897 to avoid flashing after stroke
completion can still sometimes have a flash.  Be more aggressive in the
retainment of the Ink input snapshot, and only drop it once a call to
PdfViewWebPlugin::Paint() confirms that it is not needed.

Bug: 380057101
Change-Id: Iacfe3cc29f16461fdadb7b33fccdbced9694d0b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6367699
Commit-Queue: Alan Screen <awscreen@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434635}

pdf/pdf_view_web_plugin.cc

@@ -356,6 +356,10 @@ class PdfViewWebPlugin::PdfInkModuleClientImpl : public PdfInkModuleClient {
                    InkStrokeId id,
                    const ink::Stroke& stroke) override {
     plugin_->engine_->ApplyStroke(page_index, id, stroke);
+
+    // `snapshot_` is now stale, since its content doesn't contain this newly
+    // added stroke.
+    plugin_->snapshot_needs_update_for_ink_input_ = true;
   }
 
   void StrokeFinished() override {
@@ -627,10 +631,15 @@ void PdfViewWebPlugin::Paint(cc::PaintCanvas* canvas, const gfx::Rect& rect) {
             .TakePaintImage();
     canvas->drawImage(snapshot_ink_inputs_.value(), 0, 0);
   } else if (snapshot_ink_inputs_.has_value()) {
-    // Waiting on `snapshot_` to get refreshed to reflect the change for an
-    // added stroke, so reapply the last Ink inputs snapshot to avoid a flash
-    // of a recently added stroke temporarily disappearing.
-    canvas->drawImage(snapshot_ink_inputs_.value(), 0, 0);
+    if (snapshot_needs_update_for_ink_input_) {
+      // Still waiting on `snapshot_` to get refreshed to reflect the change
+      // for an added stroke, so reapply the last Ink inputs snapshot to avoid
+      // a flash of a recently added stroke temporarily disappearing.
+      canvas->drawImage(snapshot_ink_inputs_.value(), 0, 0);
+    } else {
+      // Now safe to discard the previous Ink inputs snapshot.
+      snapshot_ink_inputs_.reset();
+    }
   }
 #endif  // BUILDFLAG(ENABLE_PDF_INK2)
 }
@@ -2276,9 +2285,8 @@ void PdfViewWebPlugin::UpdateSnapshot(sk_sp<SkImage> snapshot) {
 #if BUILDFLAG(ENABLE_PDF_INK2)
   // `paint_manager_` updates the snapshot after it has completed painting,
   // which uses `engine_` in `DoPaint()`.  Any newly added Ink stroke will now
-  // be applied in the snapshot, so there is no need to retain any prior
-  // `snapshot_ink_inputs_`.
-  snapshot_ink_inputs_.reset();
+  // be applied in the snapshot.
+  snapshot_needs_update_for_ink_input_ = false;
 #endif
 
   if (!plugin_rect_.IsEmpty())

pdf/pdf_view_web_plugin.h

@@ -775,6 +775,10 @@ class PdfViewWebPlugin final : public PDFiumEngineClient,
 #if BUILDFLAG(ENABLE_PDF_INK2)
   // The last saved image snapshot for rendering of Ink inputs.
   std::optional<cc::PaintImage> snapshot_ink_inputs_;
+
+  // Tracks if `snapshot_` still needs to be updated to reflect a newly
+  // added Ink stroke.
+  bool snapshot_needs_update_for_ink_input_ = false;
 #endif
 
   // Translate from snapshot to device pixels.

pdf/pdf_view_web_plugin_unittest.cc

@@ -3025,10 +3025,10 @@ TEST_F(PdfViewWebPluginInkTest, DrawInProgressStroke) {
   // not include the last Ink stroke.  This results in the most recent stroke
   // disappearing, causing a flash for the user unless the snapshot from the
   // most recent stroke is reused.
-  EXPECT_TRUE(plugin_->HasInkInputsSnapshotForTesting());
   plugin_->Paint(canvas_.sk_canvas(), kScreenRect);
   EXPECT_TRUE(MatchesPngFile(canvas_.GetBitmap().asImage().get(),
                              stroked_image_png_file));
+  EXPECT_TRUE(plugin_->HasInkInputsSnapshotForTesting());
 
   // Simulate how the snapshot eventually gets updated, after all necessary
   // tasks that normally happen from the PaintManager finally complete.  That
@@ -3038,10 +3038,10 @@ TEST_F(PdfViewWebPluginInkTest, DrawInProgressStroke) {
   // engine.
   plugin_->UpdateSnapshot(CreateSkiaImageForTesting(
       plugin_->GetPluginRectForTesting().size(), SK_ColorWHITE));
-  EXPECT_FALSE(plugin_->HasInkInputsSnapshotForTesting());
   plugin_->Paint(canvas_.sk_canvas(), kScreenRect);
   EXPECT_TRUE(cc::MatchesBitmap(canvas_.GetBitmap(), blank_bitmap,
                                 cc::ExactPixelComparator()));
+  EXPECT_FALSE(plugin_->HasInkInputsSnapshotForTesting());
 }
 
 class PdfViewWebPluginInk2SaveTest : public PdfViewWebPluginSaveTest {