Qt Drag and Drop across two windows of the same app is not detected

[Andrew Hayzen]

What happened:
1) Installed multi-window silo for the webbrowser [0] and installed demo app [1]
2) Launched the multi window drag and drop example app, note sometimes the windows are spawned on top of each other, so you may need to move one to see both
3) Attempt to drag one of the red boxes into the centre of the other window
4) Notice that on unity7 the centre of the window turns green whereas on unity8 nothing happens
5) Notice on unity7 that when the mouse is released the red box is moved into that window

What I expected to happen:
On unity8 for it to act the same as unity7, for the drag handle to work correctly and for the window droparea detection to work.

Note at step 3 while dragging the box that on unity7 a handle correctly appears near the mouse and follows it, whereas on unity8 it spawns a window with the image for the handle but it doesn't follow the mouse. I am assuming this is linked to other existing bugs within unity8/mir where showing menus appear in separate windows? But I'm not sure which bug that is?

I am currently assuming that either the increased security of Mir between windows is preventing the Qt drag system from being to detect when the mouse is over another window? (even though they are the same app) Or is the issue that this functionality has not been implemented yet?

I have the a xenial desktop with the stable overlay and the multi-window silo for the webbrowser [0]. The demo mini-app I have linked to this bug and can be found at [1]. Let me know if you need help running it.

The use case for this scenario, is moving tabs in the webbrowser between multiple windows and dragging a tab out of a window to spawn a new window.

0 - https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/landing-084/+packages
1 - https://code.launchpad.net/~ahayzen/+junk/multi-window-dnd-cpp-001

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Nick Dedekind (nick-dedekind)]

We do not yet have support for drag and drop between windows.