Bug: Viewer split view resets viewport position on pointer exit

[rleigh]

I found that the split view behaviour is incorrect, at least on Linux (other platforms not yet tested). This is seen for images which are big enough to require scrolling in the split view.

Select split view tab.
Initial view is centred. Hoewever, not that the y scrollbar is not centred, but x is. See the -initial screenshot.

Full zoomed out view is shown in the -all screenshot for comparison.

Try scrolling the image. See -scroll1 screenshot; in this example we have moved to the far bottom left of the split view.
Now, move the mouse outside the viewport/scrollbar area. The view resets to that shown in the -scroll2 screenshot. Notice that the view has been recentred in both x and y but the scrollbars remain unchanged.

Whatever signal is triggered on exiting the viewport, the recentre of the viewport is not desired since it is directly counter to the user's intentions of scrolling with the scrollbars.

[rleigh]

Initial view (split view is centred)

[rleigh]

Overview of entire split image (for reference)

[rleigh]

User scrolled image to bottom left

[rleigh]

View reset to centre when pointer leaves viewport; scrollbars unchanged

[jamoore]

Splitting 5.1.4 due to milestone decoupling