Import Tk 8.6.12

tests/constraints.tcl

@@ -172,6 +172,67 @@ namespace eval tk {
             return $r
         }
 
+	#
+	#  CONTROL TIMING ASPECTS OF POINTER WARPING
+	#
+	# The proc [controlPointerWarpTiming] takes care of the following timing
+	# details of pointer warping:
+	#
+	# a. Allow pointer warping to happen if it was scheduled for execution at
+	#    idle time.
+	#    - In Tk releases 8.6 and older, pointer warping is scheduled for
+	#      execution at idle time
+	#    - In release 8.7 and newer this happens synchronously and no extra
+	#      control is needed.
+	#    The namespace variable idle_pointer_warping records which of these is
+	#    the case.
+	#
+	# b. Work around a race condition associated with OS notification of
+	#    mouse motion on Windows.
+	#
+	#    When calling [event generate $w $event -warp 1 ...], the following
+	#    sequence occurs:
+	#    - At some point in the processing of this command, either via a
+	#      synchronous execution path, or asynchronously at idle time, Tk calls
+	#      an OS function* to carry out the mouse cursor motion.
+	#    - Tk has previously registered a callback function** with the OS, for
+	#      the OS to call in order to notify Tk when a mouse move is completed.
+	#    - Tk doesn't wait for the callback function to receive the notification
+	#      from the OS, but continues processing. This suits most use cases
+	#      because (usually) the notification comes quickly enough
+	#      (range: a few ms?). However ...
+	#    - A problem arises if Tk performs some processing, immediately following
+	#      up on [event generate $w $event -warp 1 ...], and that processing
+	#      relies on the mouse pointer having actually moved. If such processing
+	#      happens just before the notification from the OS has been received,
+	#      Tk will be using not yet updated info (e.g. mouse coordinates).
+	#
+	#         Hickup, choke etc ... !
+	#
+	#            *  the function SendInput() of the Win32 API
+	#            ** the callback function is TkWinChildProc()
+	#
+	#    This timing issue can be addressed by putting the Tk process on hold
+	#    (do nothing at all) for a somewhat extended amount of time, while
+	#    letting the OS complete its job in the meantime. This is what is
+	#    accomplished by calling [after ms].
+	#
+	#    ----
+	#    For the history of this issue please refer to Tk ticket [69b48f427e],
+	#    specifically the comment on 2019-10-27 14:24:26.
+	#
+	variable idle_pointer_warping [expr {![package vsatisfies [package provide Tk] 8.7-]}]
+	proc controlPointerWarpTiming {{duration 50}} {
+		variable idle_pointer_warping
+		if {$idle_pointer_warping} {
+			update idletasks ;# see a. above
+		}
+		if {[tk windowingsystem] eq "win32"} {
+			after $duration ;# see b. above
+		}
+	}
+	namespace export controlPointerWarpTiming
+
     }
 }