Some Mouse Events not Captured After Drag and Drop

[Bartimaeus-]

Edit: some of the content here incorrectly describes the behavior of what was happening. See #717

Summary
Some mouse events are not handled by most plot widgets after one or more curves are added to an empty plot widget

Setup
-Open a new plot widget (Plot 2)
-Add curve(s) to Plot 2

Issue
-After curve added to Plot 2, some mouse events (right/left/middle-click) are not handled by Plot 1 (mouse scroll wheel events are still handled ). Only the plot widget itself seems to have this issue-- the plot toolbar and plot legend are not impacted

Mouse event handling in the other open plots is restored after any of the following occurs:
-A mouse event occurs in any other part of PlotJuggler besides Plot 1's plotting area. This includes Plot 2 (which is handled)
-The zoom changes in Plot 1 using a mouse scroll wheel event
-The mouse moves over any of the green shaded region indicated in the figure below

Other Notes:
-Only verified on Windows (Linux not checked)
-Behavior is present at least back to 3.5.0 release (earlier versions not checked)

[Bartimaeus-]

Edit: some of the content here incorrectly describes the behavior of what was happening. See #717

I've figured out how to fix most of this by clearing the drag info and curves after a mouse button release event:

(snippet from canvasEventFilter() in plotwidget.cpp)

    case QEvent::MouseButtonRelease: {
      if (_dragging.mode == DragInfo::NONE)
      {
        QApplication::restoreOverrideCursor();
        return false;
      }
      _dragging.mode = DragInfo::NONE;     // new line added
      _dragging.curves.clear();    // new line added
    }

While testing this solution I found this fixes everything except for the first plotwidget. This is somehow due to _dragging.mode not getting reset to DrafInfo::NONE

To demonstrate the issue, I've added the following code:

    case QEvent::MouseButtonPress: {
      if (_dragging.mode != DragInfo::NONE)
      {
          QMessageBox::warning(qwtPlot(), "Drag in process",    // (this is the code that was added)
                               tr("Mouse button press event but there's a drag in process!"));
          return true;  // don't pass to canvas().  (this code was already present)
      }

You can see in the video below that after a drag and drop event onto the upper right plot that panning works as intended on the bottom curve. If I try to do the pan on the upper left plot (which was the first plotwidget that existed on the tab) that the warning popup message I added is triggered and the panning event doesn't occur:

plotjuggler_Issue-716_panning_debugging.mp4

This also applies to other button events