Add a test for PDF text selection rendering

Add PDFiumEngineTest.DrawTextSelectionsHelloWorld to render the blue
text selection on hello_world2.pdf with various selection ranges, sans
the text. The test renders the selection as in-memory bitmaps, and
compares them against expectations which are stored as PNGs.

Rendering only the selection is easier than rendering the PDF in its
entirety using PDFiumEngine. The out is also simpler too, but still
not consistent across platforms, since text rendering varies, so the
text bounding boxes are different. To draw only the selections, the test
has to reach into PDFiumEngine internals. To make this easier without
adding a lot of non-trivial "for testing" code to PDFiumEngine itself,
make PDFiumEngineTest a friend of PDFiumEngine.

This creates a baseline to check how PDF text selection works, and adds
missing test coverage. This is in preparation for improving PDF text
selection, as it oftentimes appear uneven.

Bug: 40448046
Change-Id: I37640f39aaa29ee693647b23f825a942ceb497b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5892210
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1360737}

pdf/pdfium/pdfium_engine.h

@@ -491,6 +491,7 @@ class PDFiumEngine : public DocumentLoader::Client, public IFSDK_PAUSE {
 
   friend class FormFillerTest;
   friend class PDFiumEngineTabbingTest;
+  friend class PDFiumEngineTest;
   friend class PDFiumFormFiller;
   friend class PDFiumTestBase;
   friend class SelectionChangeInvalidator;

pdf/pdfium/pdfium_engine_unittest.cc

@@ -13,6 +13,7 @@
 
 #include <utility>
 
+#include "base/files/file_path.h"
 #include "base/functional/callback.h"
 #include "base/hash/md5.h"
 #include "base/run_loop.h"
@@ -24,6 +25,7 @@
 #include "base/test/scoped_feature_list.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
+#include "pdf/accessibility_structs.h"
 #include "pdf/buildflags.h"
 #include "pdf/document_attachment_info.h"
 #include "pdf/document_layout.h"
@@ -34,6 +36,7 @@
 #include "pdf/test/mouse_event_builder.h"
 #include "pdf/test/test_client.h"
 #include "pdf/test/test_document_loader.h"
+#include "pdf/test/test_helpers.h"
 #include "pdf/ui/thumbnail.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -41,11 +44,15 @@
 #include "third_party/blink/public/common/input/web_keyboard_event.h"
 #include "third_party/blink/public/common/input/web_mouse_event.h"
 #include "third_party/blink/public/common/input/web_pointer_properties.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+#include "third_party/skia/include/core/SkCanvas.h"
+#include "third_party/skia/include/core/SkImage.h"
 #include "ui/events/keycodes/keyboard_codes.h"
 #include "ui/gfx/geometry/point.h"
 #include "ui/gfx/geometry/point_f.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size.h"
+#include "ui/gfx/geometry/skia_conversions.h"
 
 namespace chrome_pdf {
 
@@ -95,6 +102,12 @@ blink::WebMouseEvent CreateMoveWebMouseEventToPosition(
       .Build();
 }
 
+base::FilePath GetTextSelectionReferenceFilePath(
+    std::string_view test_filename) {
+  return base::FilePath(FILE_PATH_LITERAL("text_selection"))
+      .AppendASCII(test_filename);
+}
+
 class MockTestClient : public TestClient {
  public:
   MockTestClient() {
@@ -191,6 +204,68 @@ class PDFiumEngineTest : public PDFiumTestBase {
     }
     return available_pages;
   }
+
+  void SetSelection(PDFiumEngine& engine,
+                    uint32_t start_page_index,
+                    uint32_t start_char_index,
+                    uint32_t end_page_index,
+                    uint32_t end_char_index) {
+    engine.SetSelection({start_page_index, start_char_index},
+                        {end_page_index, end_char_index});
+  }
+
+  void DrawAndCompare(PDFiumEngine& engine,
+                      int page_index,
+                      std::string_view expected_png_filename) {
+    return DrawAndCompareImpl(engine, page_index, expected_png_filename,
+                              /*use_platform_suffix=*/false);
+  }
+
+  void DrawAndCompareWithPlatformExpectations(
+      PDFiumEngine& engine,
+      int page_index,
+      std::string_view expected_png_filename) {
+    return DrawAndCompareImpl(engine, page_index, expected_png_filename,
+                              /*use_platform_suffix=*/true);
+  }
+
+ private:
+  void DrawAndCompareImpl(PDFiumEngine& engine,
+                          int page_index,
+                          std::string_view expected_png_filename,
+                          bool use_platform_suffix) {
+    const gfx::Rect rect = engine.GetPageContentsRect(page_index);
+    ASSERT_TRUE(!rect.IsEmpty());
+
+    SkBitmap bitmap;
+    bitmap.allocPixels(
+        SkImageInfo::MakeN32Premul(gfx::SizeToSkISize(rect.size())));
+    SkCanvas canvas(bitmap);
+    canvas.clear(SK_ColorWHITE);
+
+    const size_t progressive_index = engine.StartPaint(page_index, rect);
+    CHECK_EQ(0u, progressive_index);
+    engine.DrawSelections(progressive_index, bitmap);
+    engine.progressive_paints_.clear();
+
+    base::FilePath expectation_path =
+        GetTextSelectionReferenceFilePath(expected_png_filename);
+
+#if BUILDFLAG(IS_WIN) || BUILDFLAG(IS_MAC)
+    // Note that the expectation files without a suffix is typically generated
+    // on Linux, so there is no code here to add a suffix for Linux.
+    if (use_platform_suffix) {
+#if BUILDFLAG(IS_WIN)
+      constexpr std::wstring_view kSuffix = L"_win";
+#else
+      constexpr std::string_view kSuffix = "_mac";
+#endif  // BUILDFLAG(IS_WIN)
+      expectation_path = expectation_path.InsertBeforeExtension(kSuffix);
+    }
+#endif  // BUILDFLAG(IS_WIN) || BUILDFLAG(IS_MAC)
+
+    EXPECT_TRUE(MatchesPngFile(bitmap.asImage().get(), expectation_path));
+  }
 };
 
 TEST_P(PDFiumEngineTest, InitializeWithRectanglesMultiPagesPdf) {
@@ -950,6 +1025,38 @@ TEST_P(PDFiumEngineTest, SelectLinkAreaWithNoText) {
   EXPECT_EQ(kExpectedText, engine->GetSelectedText());
 }
 
+TEST_P(PDFiumEngineTest, DrawTextSelectionsHelloWorld) {
+  constexpr int kPageIndex = 0;
+  NiceMock<MockTestClient> client;
+  std::unique_ptr<PDFiumEngine> engine =
+      InitializeEngine(&client, FILE_PATH_LITERAL("hello_world2.pdf"));
+  ASSERT_TRUE(engine);
+
+  // Update the plugin size so that all the text is visible by
+  // `SelectionChangeInvalidator`.
+  engine->PluginSizeUpdated({500, 500});
+
+  EXPECT_THAT(engine->GetSelectedText(), IsEmpty());
+  DrawAndCompare(*engine, kPageIndex, "hello_world_blank.png");
+
+  SetSelection(*engine, /*start_page_index=*/kPageIndex, /*start_char_index=*/1,
+               /*end_page_index=*/kPageIndex, /*end_char_index=*/2);
+  EXPECT_EQ("e", engine->GetSelectedText());
+  DrawAndCompare(*engine, kPageIndex, "hello_world_selection_1.png");
+
+  SetSelection(*engine, /*start_page_index=*/kPageIndex, /*start_char_index=*/0,
+               /*end_page_index=*/kPageIndex, /*end_char_index=*/3);
+  EXPECT_EQ("Hel", engine->GetSelectedText());
+  DrawAndCompareWithPlatformExpectations(*engine, kPageIndex,
+                                         "hello_world_selection_2.png");
+
+  SetSelection(*engine, /*start_page_index=*/kPageIndex, /*start_char_index=*/0,
+               /*end_page_index=*/kPageIndex, /*end_char_index=*/6);
+  EXPECT_EQ("Hello,", engine->GetSelectedText());
+  DrawAndCompareWithPlatformExpectations(*engine, kPageIndex,
+                                         "hello_world_selection_3.png");
+}
+
 TEST_P(PDFiumEngineTest, LinkNavigates) {
   NiceMock<MockTestClient> client;
   std::unique_ptr<PDFiumEngine> engine =