[rb] be more exact about expected sizes of screenshots

rb/spec/integration/selenium/webdriver/takes_screenshot_spec.rb

@@ -81,13 +81,20 @@ def save_screenshot_and_assert(source, path)
         end
 
         it 'takes viewport screenshot by default' do
+          viewport_width = driver.execute_script("return window.innerWidth;") * 2
+          viewport_height = driver.execute_script("return window.innerHeight;") * 2
+
           screenshot = driver.save_screenshot path
-          expect(File.read(screenshot)[0x10..0x18].unpack('NN').last).to be < 2600
+          expect(File.read(screenshot)[0x10..0x18].unpack('NN')).to eq([viewport_width, viewport_height])
         end
 
         it 'takes full page screenshot', exclusive: {browser: :firefox} do
+          viewport_width = driver.execute_script("return window.innerWidth;") * 2
+          viewport_height = driver.execute_script("return window.innerHeight;") * 2
+
           screenshot = driver.save_screenshot path, full_page: true
-          expect(File.read(screenshot)[0x10..0x18].unpack('NN').last).to be > 2600
+          expect(File.read(screenshot)[0x10..0x18].unpack1('NN')).to eq viewport_width
+          expect(File.read(screenshot)[0x10..0x18].unpack('NN').last).to be > viewport_height
         end
 
         it 'does not take full page screenshot', exclude: {browser: :firefox} do