Is it possible, in terms of ABI breakage, to add fields to SDL_MouseMotionEvent?
I see that d5ec735 added a field to SDL_TouchFingerEvent and the discussion at the bottom of #3037 (comment) suggests it's OK.

Given SDL_Event is 56 bytes =14 x 4-byte values, and SDL_MouseMotionEvent currently has 9, there's room for 5 more 4-byte values. e.g. this would fit:

    float xFloat;           /**< X coordinate, relative to window.
                                 Can be non-integer if the window is on a high DPI monitor with scaling. */
    float yFloat;           /**< Y coordinate, relative to window */
    float xrelFloat;        /**< The relative motion in the X direction */
    float yrelFloat;        /**< The relative motion in the Y direction */
} SDL_MouseMotionEvent;

Likewise for SDL_MouseButtonEvent, it could get:

    float xFloat;           /**< X coordinate, relative to window */
    float yFloat;           /**< Y coordinate, relative to window */
} SDL_MouseButtonEvent;

Any progress on this?

I actually did a quick prototype of this last year, for Wayland + macOS: https://github.com/ericwa/SDL/commits/float-mouse-coords
Not sure if I got all of the rounding details correct for the legacy codepaths, but at least it's a proof of concept.

Now that development on SDL3 has begun...

I think there are two options here (assuming we stick with using DPI-scaled units, which I believe we tentatively decided to do): change all mouse APIs so coordinate values are in floats, or add additional APIs for precise float coordinates alongside the current integer APIs.

I'm a fan of keeping things simple and having just a single set of APIs, but I can also imagine the mouse APIs are frequently used together with things like the window size and position APIs which don't really have good reason to have high precision float values, so there might be a lot of int <-> float conversions in people's code if mouse APIs only use floats for position.

If the extra variant approach is taken, using a Precise suffix or prefix for those names makes sense to me - something similar is done for mouse wheel events already.

Here's a list of all the existing position-related mouse APIs that will need to change or have Precise variants:

// In SDL_MouseMotionEvent:
Sint32 x
Sint32 y;
Sint32 xrel;
Sint32 yrel;

// In SDL_MouseButtonEvent:
Sint32 x;
Sint32 y;

// In SDL_MouseWheelEvent:
Sint32 mouseX;
Sint32 mouseY;

Uint32 SDL_GetMouseState(int *x, int *y);
Uint32 SDL_GetGlobalMouseState(int *x, int *y);
Uint32 SDL_GetRelativeMouseState(int *x, int *y);
void SDL_WarpMouseInWindow(SDL_Window * window, int x, int y);
int SDL_WarpMouseGlobal(int x, int y);

just a thought, not sure if this has been said: currently touch event as reported to a window (and also within the window, i think).
but:

• you can touch outside the window.
• the window can be also across two screens!

So it seems to me it should be linked to a VideoDisplay instead, (but we only have ids for them), so maybe linked to the SDL_DisplayMode of the VideoDisplay.
(probably also for mouse event too etc.)

Direct touch event coordinates are relative to the window though rather than a screen, right?

it is yes

FYI, I'm working on this now.