x11: Read mouse settings at startup so the first click is not reordered

[probonopd]

Problem

Right after an application starts, a quick click on a table row highlights the row but does not select it: no tableViewSelectionDidChange:, no action. Only the second click works. Slow clicks (button held for ~150 ms) are not affected.

Cause

The first ButtonPress calls -initializeMouse from -processEvent:, before the mouse-down event is queued. -initializeMouse registers with NSDistributedNotificationCenter, which waits for gdnc's reply in a nested run loop (NSConnectionReplyMode). The X event source is also active in that mode, so the nested loop processes the pending ButtonRelease and queues the mouse-up first. When -processEvent: returns, the mouse-down is appended after it, so the queue holds [mouse-up, mouse-down]. The table sees the mouse-down last and waits in its tracking loop for a mouse-up that was already delivered.

Backtrace of the ButtonRelease being processed inside the press (abridged):

-[XGServer(EventOps) processEvent:]              ButtonRelease
-[XGServer(EventOps) receivedEvent:type:extra:forMode:]
-[GSRunLoopCtxt pollUntil:within:]
-[NSRunLoop runMode:beforeDate:]                 NSConnectionReplyMode
-[GSMessageHandle sendMessage:beforeDate:]
-[NSConnection(Private) _sendOutRmc:type:sequence:]
...
-[NSDistributedNotificationCenter addObserver:selector:name:object:suspensionBehavior:]
-[XGServer(EventOps) initializeMouse]
-[XGServer(EventOps) processEvent:]              ButtonPress

Fix

Call -initializeMouse once from -initWithAttributes: instead of lazily on the first button press, so no press has to wait for another process.

How to reproduce

1. Save this as firstclick.m:

#import <AppKit/AppKit.h>

@interface Controller : NSObject
@end

@implementation Controller
- (NSInteger) numberOfRowsInTableView: (NSTableView *)tv
{
  return 5;
}
- (id) tableView: (NSTableView *)tv
objectValueForTableColumn: (NSTableColumn *)col
             row: (NSInteger)row
{
  return [NSString stringWithFormat: @"Row %ld", (long)row];
}
- (void) tableViewSelectionDidChange: (NSNotification *)n
{
  fprintf(stderr, "selected row %ld\n", (long)[[n object] selectedRow]);
}
- (void) applicationDidFinishLaunching: (NSNotification *)n
{
  NSWindow *w = [[NSWindow alloc]
    initWithContentRect: NSMakeRect(100, 100, 300, 200)
              styleMask: NSTitledWindowMask
                backing: NSBackingStoreBuffered
                  defer: NO];
  NSScrollView *sv = [[NSScrollView alloc] initWithFrame: [[w contentView] bounds]];
  NSTableView *tv = [[NSTableView alloc] initWithFrame: [[w contentView] bounds]];
  NSTableColumn *col = [[NSTableColumn alloc] initWithIdentifier: @"c"];

  [col setWidth: 280];
  [tv addTableColumn: col];
  [tv setHeaderView: nil];
  [tv setDataSource: self];
  [tv setDelegate: self];
  [sv setDocumentView: tv];
  [[w contentView] addSubview: sv];
  [w makeKeyAndOrderFront: nil];
  fprintf(stderr, "ready\n");
}
@end

int main(int argc, const char **argv)
{
  NSAutoreleasePool *pool = [NSAutoreleasePool new];
  NSApplication *app = [NSApplication sharedApplication];

  [app setDelegate: [Controller new]];
  [app run];
  [pool release];
  return 0;
}

2. Build it:

. /usr/GNUstep/System/Library/Makefiles/GNUstep.sh   # or wherever GNUstep.sh lives
clang $(gnustep-config --objc-flags) firstclick.m -o firstclick $(gnustep-config --gui-libs)

3. Start it, and within a few seconds of ready being printed click "Row 1" once with a very short press. xdotool produces such a click reliably (press and release arrive together); a fast real click does the same:

./firstclick &
sleep 5
xdotool mousemove <x of Row 1> <y of Row 1> click 1
sleep 1
xdotool click 1

Without this patch:

ready
selected row 1      <- only after the second click

With this patch:

ready
selected row 1      <- after the first click

The bug needs gdnc to be involved, i.e. the first ButtonPress must trigger the distributed notification registration, so always test on a freshly started process.

Tested on Linux (X11, cairo backend): in repeated runs the first click was lost every time without the patch and never with it. With the patch, double-click detection and the mouse defaults (GSDoubleClickTime, GSMouseMoveThreshold, GSMouseScrollMultiplier, GSMenuButtonEnabled/GSMenuButtonEvent) are read at startup and still follow later changes through the same NSUserDefaultsDidChangeNotification observer.

cc @pkgdemon