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Diffstat (limited to 'tests/constraints.tcl')
| -rw-r--r-- | tests/constraints.tcl | 61 |
1 files changed, 61 insertions, 0 deletions
diff --git a/tests/constraints.tcl b/tests/constraints.tcl index ee073cf..65609d6 100644 --- a/tests/constraints.tcl +++ b/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 + } } |