Make file-picker modal in Chromium for Linux

When I started working on Chromium, I found a bug in Chromium for Linux: the file-picker was not modal. So, while a file-picker was open, the user could still control the main window. Sometimes this caused misbehavior — for example, you could send an email while still attaching a file.

There was already a bug filed and it seemed easy to fix, but it took two years to fully resolve.

The root cause is as follows.

Chromium for Linux uses GtkFileChooserDialog to open a file-picker, but it is not modal to the X11 host window because GtkFileChooserDialog can only be modal to its parent GtkWindow. So I tried to let the X11 host window disable input event handling to make the file-picker modal. Here are the details.

Opening a file-picker

DisableEventListening() disables event listening for the host window using aura::ScopedWindowTargeter, which temporarily replaces the event targeter with ui::NullEventTargeter. It returns a scope handle that is used to call destroy_callback when the file-picker closes.

class ScopedHandle {
 public:
  explicit ScopedHandle(const base::Closure& destroy_callback);
  ~ScopedHandle();
  void CancelCallback();

 private:
  base::Closure destroy_callback_;
  DISALLOW_COPY_AND_ASSIGN(ScopedHandle);
};

In addition, we also set another destroy callback (OnFilePickerDestroy) on the GtkFileChooserDialog that is called when the file-picker is closed.

Closing the file-picker

As you can see, OnFilePickerDestroy deletes scoped_handle.

void OnFilePickerDestroy(views::DesktopWindowTreeHostX11::ScopedHandle*
                             scoped_handle) {
  delete scoped_handle;
}

Then the destroy_callback of ScopedHandle below is automatically called.

void DesktopWindowTreeHostX11::EnableEventListening() {
  DCHECK(modal_dialog_xid_);
  modal_dialog_xid_ = 0;
  targeter_for_modal_.reset();
}

You can find more details and discussion in this design doc.

The first change list was reverted due to a UI-freezing problem that happened when the user opened a file-picker from a child window of the X11 host window. The second change list finally fixed this issue (BUG 408481, 579408). I also added a test case for the fix: BrowserSelectFileDialogTest.ModalTest.