Fix inactive window passes events to window below (#4561, #4835)

Prior to this fix, if all of the following conditions were met:

While a modal window (such as the Preferences dialog) was open.
‘Multiple window UI’ was enabled.
A non-modal window (such as the Preview window) overlapped the modal window.
The non-modal window was selected, so this window is on top of the modal window. Caused mouse clicks and movements within the overlay area of the non-modal window to be incorrectly passed to the window below, i.e. the modal window. Instead, it would be expected that no action would be taken.

@dacap note: At the moment this fix only works on macOS because the os::WindowSpec::modal() parameter is only used on macOS.

Aseprite

Introduction

Aseprite is a program to create animated sprites. Its main features are:

Sprites are composed of layers & frames as separated concepts.
Support for color profiles and different color modes: RGBA, Indexed (palettes up to 256 colors), Grayscale.
Animation facilities, with real-time preview and onion skinning.
Export/import animations to/from sprite sheets, GIF files, or sequence of PNG files (and FLC, FLI, JPG, BMP, PCX, TGA).
Multiple editors support.
Layer groups for organizing your work, and reference layers for rotoscoping.
Pixel-art specific tools like Pixel Perfect freehand mode, Shading ink, Custom Brushes, Outlines, Wide Pixels, etc.
Other special drawing tools like Pressure sensitivity, Symmetry Tool, Stroke and Fill selection, Gradients.
Tiled mode useful to draw patterns and textures.
Transform multiple frames/layers at the same time.
Lua scripting capabilities.
CLI - Command Line Interface to automatize tasks.
Quick Reference / Cheat Sheet keyboard shortcuts (customizable keys and mouse wheel).
Reopen closed files and recover data in case of crash.
Undo/Redo for every operation and support for non-linear undo.
More features & tips

There is a list of Known Issues (things to be fixed or that aren’t yet implemented).

If you found a bug or have a new idea/feature for the program, you can report them.

You can ask for help in:

Aseprite was originally created by David Capello and is now being developed and maintained by Igara Studio and contributors.

Check the AUTHORS file for details about the active team of developers working on Aseprite.

This program is distributed under three different licenses:

Source code and official releases/binaries are distributed under our End-User License Agreement for Aseprite (EULA). Please check that there are modules/libraries in the source code that are distributed under the MIT license (e.g. laf, clip, undo, observable, ui, etc.).
You can request a special educational license in case you are a teacher in an educational institution and want to use Aseprite in your classroom (in-situ).
Steam releases are distributed under the terms of the Steam Subscriber Agreement.

You can get more information about Aseprite license in the FAQ.