In Qt backend of imshow(), the image shown in the window can be zoomed and dragged with the mouse. And the function DefaultViewPort::controlImagePosition() aims to keep the image border out of the visible window area. However, this function doesn't work properly when the image is zoomed (the translation distance should be scaled by current zooming factor), which makes it hard to check the pixel value near image borders, see the gif below:

This PR resolves the problem:

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=Custom
build_image:Custom=qt:16.04
buildworker:Custom=linux-1,linux-2,linux-4,linux-6