Fix delayed reaction to USR1/2 signals

While using xmobar to reproduce/fix a bug in xmonad I noticed that
xmobar doesn't react immediately to the signals to reposition or change
screen.

Apparently none of the Xlib functions called from `repositionWin` flush
the Xlib output buffer (XMoveResizeWindow is known to not flush,
although it's unfortunately not documented), so when compiled with the
threaded runtime that runs XNextEvent in a separate thread, the
reposition is sent to the X server only later when other stuff happens.
With all monitors set to refresh once a minute, this can take a looong
time.

(It's entirely possible this happens even without the threaded runtime,
I didn't test that, sorry.)

The fix is to call XSync at the end of `repositionWin`.

While at it, I spotted that drawInWin calls XSync with discard=true,
which seems like a mistake. We don't want to discard any events, do we?
(In practice, I'd expect the `eventer` thread to read all events even
before the drawing thread manages to discard them, so even if this
discarding XSync ever had any reason to be there, I don't think it does
anything meaningful these days. I may be wrong, though.)

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.md b/changelog.md
index 072932e..0f24ebf 100644
--- a/changelog.md
+++ b/changelog.md
@@ -8,6 +8,7 @@ _Bug fixes_
   - Revert the double-UTF-8 encoding workarounds of 0.36 (#482), as they're no
     longer necessary with xmonad-contrib master, and aren't needed with any
     released version of xmonad-contrib either.
+  - Fix slow reactions to SIGUSR1/2 signals (reposition, change screen).
 
 ## Version 0.37 (November, 2020)