I found the cause in my case: X-Mouse Button Control (XMBC). With XMBC running, Minecraft versions using the new SDL3 input backend were duplicating local mouse input in relative mouse mode. This caused issues such as doors opening/closing immediately, slabs being placed twice, inventory clicks behaving incorrectly, and the mouse wheel skipping two hotbar slots at a time.
After completely closing/removing XMBC, the problem disappeared immediately. The same PC, mouse, Windows installation and Minecraft version now work normally.
So if anyone has similar mouse/input issues after the switch to SDL3, please check whether X-Mouse Button Control is installed or running. It seems XMBC does not interact correctly with the new SDL3 input path / relative mouse mode.
Francesca
I found the cause in my case: X-Mouse Button Control (XMBC).
With XMBC running, Minecraft versions using the new SDL3 input backend were duplicating local mouse input in relative mouse mode. This caused issues such as doors opening/closing immediately, slabs being placed twice, inventory clicks behaving incorrectly, and the mouse wheel skipping two hotbar slots at a time.
After completely closing/removing XMBC, the problem disappeared immediately. The same PC, mouse, Windows installation and Minecraft version now work normally.
So if anyone has similar mouse/input issues after the switch to SDL3, please check whether X-Mouse Button Control is installed or running. It seems XMBC does not interact correctly with the new SDL3 input path / relative mouse mode.