@tacaswell @kuch3n A little background: The code in backend_wx.py has many calls to self.CaptureMouse() in the "MouseButton Down" events and if self.HasCapture(): self.ReleaseMouse() in the "MouseButton Up" events.

It's not clear to me why this is necessary (or desirable), and it seems to cause the C++ assertions that @kuch3n referred to, especially on Linux and OS X.

From https://www.wxpython.org/docs/api/wx.Window-class.html#CaptureMouse (and also https://wxpython.org/Phoenix/docs/html/wx.Window.html#wx.Window.CaptureMouse)
wxWidndow.CaptureMouse() is intended to force all mouse events to the window that captured the mouse, even if focus is lost to another window or application. I don't know why a matplotlib cakend should want to do this.

FWIW, In wxmplot we just monkey-patch CaptureMouse to be a no-op method for Linux and OSX, and mouse actions work fine, without these C++ assertions from leaving and re-entering the Window.

The docs pointed to say:

Any application which captures the mouse in the beginning of some operation
must handle wx.MouseCaptureLostEvent and cancel this operation when it
receives the event. The event handler must not recapture mouse.

I'm pretty sure that backend_wx.py does not handle wx.MouseCaptureLostEvent, as would be required. That probably explains the assertion error.

FWIW, In wxmplot we just monkey-patch CaptureMouse to be a no-op method for Linux and OSX, and mouse actions work fine, without these C++ assertions from leaving and re-entering the Window. So IMHO, the CaptureMouse() is doing more harm than good.

I'd be willing to make a PR removing CaptureMouse()/ReleaseMouse() if desired.

Uh oh!

WX mouseevent assertion errors #7636

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