From: Hongchao Liu <liuhongchao@xiaomi.com>

Resend of an earlier RFC [1] which unfortunately reached qemu-devel
with a GB2312 charset (due to a corporate e-mail footer my previous
mailer silently appended in Chinese), so patchew could not parse the
patch ("error: cannot convert from gb2312 to UTF-8"). This v2 is the
same code change, re-sent via a mailer that preserves UTF-8 and does
not inject any footer; no source changes relative to v1.

v1: https://lists.nongnu.org/archive/html/qemu-devel/2026-05/msg01701.html

Summary (unchanged from v1):

SDL_SetRelativeMouseMode(SDL_TRUE), which ui/sdl2.c currently enables
on the grab paths via sdl_hide_cursor(), switches SDL on X11/XInput2
to consume only XI_RawMotion events. X servers that can only inject
MotionNotify (VNC via XTestFakeMotionEvent, nested X sessions, some
remote desktop setups) therefore never deliver input to the guest once
it enters a relative-pointer grab. The GTK backend in QEMU already
side-steps the problem by doing manual warp-to-center in gd_motion_event
(ui/gtk.c); this patch applies the same pattern to the SDL2 backend.

Points I would especially like feedback on:

 1. Keeping sdl_hide_cursor() around with G_GNUC_UNUSED vs just
    deleting it. I kept it for symmetry with sdl_show_cursor() and so
    the XI2 rationale has an obvious place to live; I have no strong
    preference.

 2. Whether the edge-detection threshold (<= 0, >= size - 1) is
    agreeable given SDL_SetWindowGrab clamping semantics.

 3. Any concerns about the default-behavior change for local-X11
    users. In my testing, local-X11 behavior is equivalent: the host
    pointer just gets warped by a different code path, while xrel/yrel
    delivered to the guest are unchanged.

Tested on:
 - Linux/X11 (local Xorg), SDL2 2.30.x, qemu-system-arm with a
   relative virtio-mouse: pointer is no longer stuck at the window
   edge in grab mode; release after quick drag delivers correctly;
   absolute (virtio-tablet) path unchanged.
 - Same host reached via VNC: no regression; mouse input stays
   functional inside grab.

Not tested: macOS Cocoa, Windows, Wayland-only setups.

Hongchao Liu (1):
  ui/sdl2: replace SDL relative mouse mode with manual warp-to-center

 ui/sdl2.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 61 insertions(+), 3 deletions(-)

-- 
2.34.1