The patch fixes the problem here, and I didn't notice regressions either so far.
I only did a quick test though, and cannot really comment on the patch per se.

One thing maybe: with the Desktop layout, if you switch an applet to edit mode (by pressing and holding the left mouse button), it's not possible to exit edit mode by left-clicking on the desktop (even if no left-click action is configured). You need to click on a different applet now, but what if there is none...

Without this patch it works.

You can try it like that:

if (!m_editMode) {
    event->setAccepted(false);
}

or

event->setAccepted(m_editMode);

In D24748#552313, @anthonyfieroni wrote:

You can try it like that:

if (!m_editMode) {
    event->setAccepted(false);
}

or

event->setAccepted(m_editMode);

That doesn't help, unfortunately.

This patch fixes the bug, but it does indeed break leaving per-widget edit mode. Could we also exit that on left-click regardless of what other action it performs?