[3.x] Backport nonexclusive fullscreen mode.

[bruvzg]

Adds equivalent of 4.x non-exclusive fullscreen mode, that forces composition to be enabled, allowing multiple windows on the screen and interactions with other apps always work. Mode added as a separate use_nonexclusive_fullscreen property that work in conjunction with existing fullscreen property (since there's no mode concept in 3.x).

Fixes #107140

[lawnjelly]

Note that I can review to an extent, but it would benefit from testing / review by contributors that have windows (I have no windows machines, and haven't used windows for 8 years or so, so am not up to date on that OS).

[bruvzg]

For the reference, current 4.x implementation PR - #88852

Ideally, it should be tested on all vendor GPUs (I have done only limited tests with one NVIDIA and one integrated AMD GPUs).

[lawnjelly]

While this works, it seems a pity that all this wasn't done as a bitfield, with an enum for setting / disabling the various flags (especially as this one seems windows only for now). If we need in the future to increase this number of flags (to deal with e.g. special modes on Android with slots etc), and adding specific setter / getters to OS for each is kind of messy.

Whether this can be changed at this stage and maintaining backward compatibility I'm not sure.

We could switch to a bitfield internal system, and add the new functionality by using enums, e.g.:

enum DisplayFeature
{
DF_MAXIMIZED,
DF_RESIZABLE,
DF_BORDERLESS,
DF_NON_EXCLUSIVE_FULLSCREEN,
};

uint32_t _display_features = 0;

OS::set_display_feature(DisplayFeature p_feature, bool p_enable)
{
if (p_enable)
{
_display_features |= p_feature;
}
else
{
_display_feature &= ~p_feature;
}
... etc
}

Have the existing functions work through this backend, and mark them deprecated or something.

Depends what others think. Maybe on the other hand we are too far down this road to bother changing in 3.x. 🤷

[bruvzg]

4.x system with separate mode enum (windowed, minimized, maximized, fullscreen, since all are mutually exclusive) and flags bitfield (resizable, borderless) make more sense. But I'm not how many changes in 3.x we should introduce, it's probably better to keep API fully backward compatible.