[SDL3] Window used in plugin as child of host will lose mouse events on macOS

[kunitoki]

I have a small framework built on top of SDL2 which can target audio plugins editor windows.

I tried porting the framework to SDL3 but plugin windows (standalone windows work fine) but on Cocoa (macOS Sequoia), when i open the plugin window from the host and click the slider inside the window and drag it (without making the cursor to leave the window), an immediate SDL_SetMouseFocus(NULL) is generated because SDL is detecting we are outside of the window (which we are not!) and then any mouse interaction with the window is failing badly.

Seems like this is not doing the right detection:

void Cocoa_HandleMouseEvent(SDL_VideoDevice *_this, NSEvent *event)
{
    // ...

    // All events except NSEventTypeMouseExited can only happen if the window
    // has mouse focus, so we'll always set the focus even if we happen to miss
    // NSEventTypeMouseEntered, which apparently happens if the window is
    // created under the mouse on macOS 12.7
    NSEventType event_type = [event type];
    if (event_type == NSEventTypeMouseExited) {
        Cocoa_MouseFocus = NULL;
    } else {
        Cocoa_MouseFocus = [event window];
    }

The exact same plugin works fine in SDL2 (but i don't receive the mouse enter and leave events).

[slouken]

Can you provide a simple standalone example so we can debug this?

[kunitoki]

Yeah i can give you a branch and instructions to try. Seems like in SDL3 events generated on the host message thread are not forwarded to event watchers (not event filter) directly. I assume i will have to run a separate thread pumping events but i will end up having 2 message threads (host and plugins).

[kunitoki]

Just to provide more information, when using SDL2 and events generates from NSApplication, they get routed to SDL which will forward to EventWatcher which will pick it up.

This doesn't seems to be the case anymore using SDL3, no event is routed to EventWatchers when they originate from NSApplication, and that Cocoa_MouseFocus pointer seems to be the culprit in at least events that involve the mouse.

I suspect this is because when running as plugin, i'm not driving the SDL message pump anymore, but i don't know if it will be a good idea to spin another thread to do it since i will have now then 2 main UI threads, one from the host and one from SDL. Using a timer from the host to drive the message pump has proven to be unreliable, jaggy and slow overall.

[kunitoki]

To test SDL2:

git clone https://github.com/kunitoki/yup.git
cd yup
cmake -G Xcode -B build
open build/yup.xcodeproj
// select `example_plugin_clap_plugin` and build it
// launch Reaper and drag the plugin in a track, open the GUI editor 

To test SDL3

git clone https://github.com/kunitoki/yup.git
cd yup
git checkout -b dev/sdl3 origin/dev/sdl3
cmake -G Xcode -B build
open build/yup.xcodeproj
// select `example_plugin_clap_plugin` and build it
// launch Reaper and drag the plugin in a track, open the GUI editor 

You can also try building and launching example_plugin_standalone_plugin which will spawn a standalone app.