BufferView: test and fix scrolling through super long lines

test/src/test_BufferView.cc

@@ -633,3 +633,52 @@ TEST(BufferViewTest, scrolls_the_view_up_and_down_with_nonzero_scroll_offset)
     EXPECT_EQ(3, LineNumber(iterator));
     EXPECT_EQ(3, bv.cursor_screen_row());
 }
+
+TEST(BufferViewTest, scrolls_window_through_a_very_long_single_line_file)
+{
+    auto b = std::make_shared<Buffer>();
+    auto iterator = b->add_cursor();
+    for (int row = 0; row < 120; row++)
+    {
+        for (int col = 0; col < 10; col++)
+        {
+            b->insert_code_point(*iterator, (uint32_t)(row + 1000));
+        }
+    }
+    iterator->go_start_of_line();
+    BufferView bv(b, iterator, Cwd);
+    bv.resize(10, 10);
+    bv.update();
+    EXPECT_EQ(0, bv.cursor_screen_row());
+    EXPECT_EQ(1000u, **iterator);
+    for (uint32_t i = 0u; i < 100u; i++)
+    {
+        bv.scroll_view_down(1, false);
+        bv.update();
+        EXPECT_EQ(0, bv.cursor_screen_row());
+        EXPECT_EQ(i + 1001u, **iterator);
+    }
+    bv.scroll_view_down(20, false);
+    bv.update();
+    EXPECT_EQ(0, bv.cursor_screen_row());
+    EXPECT_EQ(1110u, **iterator);
+
+    for (int i = 0; i < 9; i++)
+    {
+        bv.scroll_view_up(1, false);
+        bv.update();
+        EXPECT_EQ(i + 1, bv.cursor_screen_row());
+        EXPECT_EQ(1110u, **iterator);
+    }
+    for (uint32_t i = 0; i < 50; i++)
+    {
+        bv.scroll_view_up(1, false);
+        bv.update();
+        EXPECT_EQ(9, bv.cursor_screen_row());
+        EXPECT_EQ(1109u - i, **iterator);
+    }
+    bv.scroll_view_up(1000u, false);
+    bv.update();
+    EXPECT_EQ(9, bv.cursor_screen_row());
+    EXPECT_EQ(1009u, **iterator);
+}