Save a screenshot when raising a ClickFailed exception

* Prints the path to the file
* Make it easier to debug what was actually clicked

spec/integration/session_spec.rb

@@ -415,12 +415,15 @@ describe Capybara::Session do
         two.style.top = '0px';
       JS
 
-      lambda {
+      expect {
         subject.find(:css, '#one').click
-      }.should raise_error(
-        Capybara::Webkit::ClickFailed,
-        /Failed.*\[@id='one'\].*overlapping.*\[@id='two'\].*at position/
-      )
+      }.to raise_error(Capybara::Webkit::ClickFailed) { |exception|
+        exception.message.should =~ %r{Failed.*\[@id='one'\].*overlapping.*\[@id='two'\].*at position}
+        screenshot_pattern = %r{A screenshot of the page at the time of the failure has been written to (.*)}
+        exception.message.should =~ screenshot_pattern
+        file = exception.message.match(screenshot_pattern)[1]
+        File.exist?(file).should be_true
+      }
     end
 
     it 'raises an error if a checkbox is obscured when checked' do

src/JavascriptInvocation.cpp

@@ -139,3 +139,14 @@ void JavascriptInvocation::keypress(QChar key) {
   event = QKeyEvent(QKeyEvent::KeyRelease, keyCode, Qt::NoModifier, key);
   QApplication::sendEvent(m_page, &event);
 }
+
+const QString JavascriptInvocation::render(void) {
+  QString pathTemplate =
+    QDir::temp().absoluteFilePath("./click_failed_XXXXXX.png");
+  QTemporaryFile file(pathTemplate);
+  file.open();
+  file.setAutoRemove(false);
+  QString path = file.fileName();
+  m_page->render(path, QSize(1024, 768));
+  return path;
+}

src/JavascriptInvocation.h

@@ -24,6 +24,7 @@ class JavascriptInvocation : public QObject {
     Q_INVOKABLE QVariantMap clickPosition(QWebElement element, int left, int top, int width, int height);
     Q_INVOKABLE void hover(int absoluteX, int absoluteY);
     Q_INVOKABLE void keypress(QChar);
+    Q_INVOKABLE const QString render(void);
     QVariant getError();
     void setError(QVariant error);
     InvocationResult invoke(QWebFrame *);

src/capybara.js

@@ -376,6 +376,7 @@ Capybara.ClickFailed = function(expectedPath, actualPath, position) {
     this.message += ' at position ' + position["absoluteX"] + ', ' + position["absoluteY"];
   else
     this.message += ' at unknown position';
+  this.message += "; \nA screenshot of the page at the time of the failure has been written to " + CapybaraInvocation.render();
 };
 Capybara.ClickFailed.prototype = new Error();
 Capybara.ClickFailed.prototype.constructor = Capybara.ClickFailed;