[Android] RetroArch Doesn't Ignore S Pen Hover Actions

Description

RetroArch seemingly makes no distinction between S Pen hover actions and mouse actions, resulting in S Pen functionality within RetroArch being inaccurate.

Expected behavior

When using the S Pen on a supported device (e.g., Galaxy Note Series up to Note 20, Galaxy S Series for S21 Ultra and up, etc.), hovering should do nothing (or move an unseen cursor exactly to where the S Pen is hovering over), and tapping the screen should act identical to using a finger.

Actual behavior

The S Pen hover action moves an unseen cursor on screen that starts at an unknown point to the user, and following taps from the user are registered at the point where the unseen cursor is, rather than where the S Pen actually is tapping on screen.

Steps to reproduce the bug

1. Use a S Pen supporting Samsung device
2. Go into Samsungs settings and enable Advanced Settings > S Pen > Air view > Previews and pointer
3. Open a game with the MelonDS core
4. Open MelonDS's core options and enable Screen > Touch Mode > Mouse
5. Observe failed behaviour of black cursor vs S Pen location

It is worth noting the bug does not just occur within MelonDS, as it persists throughout the entire UI. MelonDS just offers the best way of visualising how the bug is happening.

Visual produced via MelonDS

Here you can see the S Pen's location (as a hovering action) in actual space be indicated by the white arrow. The black square pixels is where the "unseen" cursor actually is within the emulator. The cursor will register clicks when the user taps the screen (indicated by the white arrow turning into a circle), but not at where the S Pen actually is, and instead only at wherever the black pixels are.

To clarify, the S Pen is never touching the screen in this example except for when the cursor lights up in a circle.

https://github.com/libretro/RetroArch/assets/54557728/7d76da31-07c1-4596-b241-87de618831a4

Additional Notes

Searching through the repository, I can see that stylus support was added in Version 1.7.7. I am unsure if the backend that Samsung uses for stylus's is different than Android core's. I also note that in Android's input drivers, there is a line to exclude stylus input, and to only include touch/mouse input.

https://github.com/libretro/RetroArch/blob/62e4779fee8aaa29a0fc216a79b5f4d5d13f6061/input/drivers/android_input.c#L1418-L1422

Nothing seems to indicate within RetroArch that I have a mouse connected but, nevertheless, the S Pen seems to roughly act as how one would expect a mouse to operate. My rough guess is here that the S Pen's backend is different than other stylus backend on Android, and that it misses this else/if, and gets interpreted as a mouse. That's just my guess as to what's happening however, I am unfamiliar with both the backend for stylus input and s pen input on Android.

Version/Commit

• RetroArch: 1581ece

Environment information

• OS: Android 13, One UI 5.1
• Device: Note 20 Ultra (SM-N986U1)

Start with input/drivers/android_input.c around lines 1418-1422, then reproduce on the Note 20 Ultra with Android 13, Air view enabled, and MelonDS Touch Mode set to Mouse. Compare hover and tap handling across the RetroArch UI. Done means S Pen hovering no longer causes an unseen mouse cursor, while screen taps register at the actual touch location.