Package: libsdl2-2.0-0
Version: 2.0.0+dfsg1-2
I've noticed that one mouse button, which was fine with SDL 1.2, didn't
appear to be working when using SDL 2.0.
Some digging showed that it's a consequence of two things: how SDL2 detects
mouse wheel usage, and a kernel patch to work around a problem with the mouse
in question regarding that button.
Since the mouse does not generate a release event when that button is
released (this appears to be by design), release events need to be
synthesised in order to avoid confusing X into thinking that a drag operation
is in progress when that button is pressed. This causes press time == release
time, and tricks SDL2 into handling it as a wheel – but, since SDL2 ‘knows’
elsewhere that Button4 and Button5 are the only possible wheel buttons, the
event is effectively discarded (it is sent with wheel.y = 0 and the button
information is lost).
(While investigating this, I noticed that the horizontal wheel ISN'T picked
up as a mouse wheel and, consequently, works without problems with SDL2 – it
fails the press time == release time test so doesn't fall foul of SDL2's X11
code only knowing about the vertical scroll wheel.)
--
| _ | Darren Salt, using Debian GNU/Linux (and Android)
| ( ) |
| X | ASCII Ribbon campaign against HTML e-mail
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You will find many bugs today.
# User Darren Salt <devs...@moreofthesa.me.uk>
# Date 1379621782 -3600
# Thu Sep 19 21:16:22 2013 +0100
Work around a false-positive in the X11 mouse wheel code
This false positive occurs when one particular button on my mouse is
pressed. The kernel which I'm using is patched to cause a release event to
be synthesised immediately when the mouse says that this button is pressed
because the mouse doesn't signal release until the button is next pressed.
(Also documents a false negative, observed with the horizontal scroll wheel
on the same mouse.)
diff -ur libsdl2-2.0.0+dfsg1.orig/src/video/x11/SDL_x11events.c libsdl2-2.0.0+dfsg1/src/video/x11/SDL_x11events.c
--- libsdl2-2.0.0+dfsg1.orig/src/video/x11/SDL_x11events.c 2013-09-19 21:36:30.000000000 +0100
+++ libsdl2-2.0.0+dfsg1/src/video/x11/SDL_x11events.c 2013-09-19 21:34:59.582201080 +0100
@@ -135,7 +135,9 @@
XPointer arg)
{
XEvent *event = (XEvent *) arg;
+ /* we only handle buttons 4 and 5 - false positive avoidance */
if (chkev->type == ButtonRelease &&
+ (event->xbutton.button == Button4 || event->xbutton.button == Button5) &&
chkev->xbutton.button == event->xbutton.button &&
chkev->xbutton.time == event->xbutton.time)
return True;
@@ -150,7 +152,12 @@
however, mouse wheel events trigger a button press and a button release
immediately. thus, checking if the same button was released at the same
time as it was pressed, should be an adequate hack to derive a mouse
- wheel event. */
+ wheel event.
+ However, there is broken and unusual hardware out there...
+ - False positive: a button for which a release event is
+ generated (or synthesised) immediately.
+ - False negative: a wheel which, when rolled, doesn't have
+ a release event generated immediately. */
if (XCheckIfEvent(display, &relevent, X11_IsWheelCheckIfEvent,
(XPointer) event)) {
