Problem with keeping aspect ratio in window when using macOS aspect-conserving resize

[radzo73]

Godot version:
3.2.1

OS/device including version:
mac OS X 10.11.16, OpenGL ES 3.0 Renderer: ATI Radeon HD 5870 OpenGL Engine

Issue description:
(Preface: I have the window resolutuion set to 256x192, display.window/stretch/mode is set to Viewport, and the Aspect is set to Keep)

In Mac OS X, you can resize a window while keeping its aspect ratio by clicking and dragging while holding Shift. However, The game begins to generate black bars as though I am not keeping the ratio, despite me doing so. It can be seen both in enlarging the window, and reducing it.

(black bars made cyan for your convenience)

Steps to reproduce:
Set your window to any size, Set your mode to Viewport, aspect to Keep, and resize with ratio. Note that resizing the window through a script will keep the correct ratio.

Minimal reproduction project:
Dawn.zip