Tcl/Tk macOS README
----------------------

This is the README file for the macOS/Darwin version of Tcl/Tk.

1. Where to go for support
--------------------------

- The tcl-mac mailing list on sourceforge is the best place to ask questions
specific to Tcl & Tk on macOS:
	http://lists.sourceforge.net/lists/listinfo/tcl-mac
(this page also has a link to searchable archives of the list, please check them
before asking on the list, many questions have already been answered).

- For general Tcl/Tk questions, the newsgroup comp.lang.tcl is your best bet:
	http://groups.google.com/group/comp.lang.tcl/

- The Tcl'ers Wiki also has many pages dealing with Tcl & Tk on macOS, see
	http://wiki.tcl.tk/_/ref?N=3753
	http://wiki.tcl.tk/_/ref?N=8361

- Please report bugs with Tk on macOS to the tracker:
	http://core.tcl.tk/tk/reportlist

2. Using Tcl/Tk on macOS
---------------------------

- There are two versions of Tk available on macOS: TkAqua using the native
aqua widgets and look&feel, and TkX11 using the traditional unix X11 wigets.
TkX11 requires an X11 server to be installed, such as Apple's X11 (which is
available as an optional or default install on recent macOS).
TkAqua and TkX11 can be distinguished at runtime via [tk windowingsystem].

- At a minimum, macOS 10.3 is required to run Tcl and TkX11.
TkAqua requires macOS 10.6 or later.

- Unless weak-linking is used, Tcl/Tk built on macOS 10.x will not run on
10.y with y < x; on the other hand Tcl/Tk built on 10.y will always run on 10.x
with y <= x (but without any of the fixes and optimizations that would be
available in a binary built on 10.x).
Weak-linking is available on OS X 10.2 or later, it additionally allows Tcl/Tk
built on 10.x to run on any 10.y with x > y >= z (for a chosen z >= 2).

- Wish checks the Resources/Scripts directory in its application bundle for a
file called AppMain.tcl, if found it is used as the startup script and the
Scripts folder is added to the auto_path. This can be used to emulate the old
OS9 TclTk droplets.

- If standard input is a special file of zero length (e.g. /dev/null), Wish
brings up the Tk console window at startup. This is the case when double
clicking Wish in the Finder (or using 'open Wish.app' from the Terminal).

- Tcl extensions can be installed in any of:
	$HOME/Library/Tcl /Library/Tcl /System/Library/Tcl
	$HOME/Library/Frameworks /Library/Frameworks /System/Library/Frameworks
	(searched in that order).
Given a potential package directory $pkg, Tcl on OSX checks for the file
$pkg/Resources/Scripts/pkgIndex.tcl as well as the usual $pkg/pkgIndex.tcl.
This allows building extensions as frameworks with all script files contained in
the Resources/Scripts directory of the framework.

- [load]able binary extensions can linked as either ordinary shared libraries
(.dylib) or as MachO bundles (since 8.4.10/8.5a3); bundles have the advantage
that they are [load]ed more efficiently from a tcl VFS (no temporary copy to the
native filesystem required).

- The 'deploy' target of macosx/GNUmakefile installs the html manpages into the
standard documentation location in the Tcl/Tk frameworks:
	Tcl.framework/Resources/Documentation/Reference/Tcl
	Tk.framework/Resources/Documentation/Reference/Tk
No nroff manpages are installed by default by the GNUmakefile.

- The Tcl and Tk frameworks can be installed in any of the system's standard
framework directories:
	$HOME/Library/Frameworks /Library/Frameworks /System/Library/Frameworks

- ${prefix}/bin/wish8.x is a script that calls a copy of 'Wish' contained in
	Tk.framework/Resources

- if 'Wish' is started from the Finder or via 'open', $argv may contain a
"-psn_XXXX" argument. This is the process serial number, you may need to filter
it out for cross platform compatibility of your scripts.

- the env array is different when Wish is started from the Finder (i.e. via
LaunchServices) than when it (or tclsh) is invoked from the Terminal, in
particular PATH may not be what you expect. (Wish started by LaunchServices
inherits loginwindow's environment variables, which are essentially those set in
$HOME/.MacOSX/environment.plist, and are unrelated to those set in your shell).

- TkAqua drawing is antialiased by default, but (outline) linewidth can be used
to control whether a line/shape is drawn antialiased. The antialiasing threshold
is 0 by default (i.e. antialias everything), it can be changed by setting
	set tk::mac::CGAntialiasLimit <limit>
in your script before drawing, in which case lines (or shapes with outlines)
thinner than <limit> pixels will not be antialiased.

- Text antialiasing by default uses the standard OS antialising settings.
Setting the global variable '::tk::mac::antialiasedtext' allows to control text
antialiasing from Tcl: a value of 1 enables AA, 0 disables AA and -1 restores
the default behaviour of respecting the OS settings.

- Scrollbars: There are two scrollbar variants in Aqua, normal & small. The
normal scrollbar has a small dimension of 15, the small variant 11.
Access to the small variant was added in Tk 8.4.2.

- The default metrics of native buttons, radiobuttons, checkboxes and
menubuttons in the Cocoa-based Tk 8.5.7 and later preserve compatibility with
the older Carbon-based implementation, you can turn off the compatibility
metrics to get more native-looking spacing by setting:
	set tk::mac::useCompatibilityMetrics 0

- TkAqua provides access to native OS X images via the Tk native bitmap facility
(including any image file readable by NSImage). A native bitmap name is
interpreted as follows (in order):
    - predefined builtin 32x32 icon name (stop, caution, document, etc)
    - name defined by [tk::mac::iconBitmap]
    - NSImage named image name
    - NSImage url string
    - 4-char OSType of IconServices icon
the syntax of [tk::mac::iconBitmap] is as follows:
	tk::mac::iconBitmap name width height -kind value
where -kind is one of
    -file	    icon of file at given path
    -fileType	    icon of given file type
    -osType	    icon of given 4-char OSType file type
    -systemType	    icon for given IconServices 4-char OSType
    -namedImage	    named NSImage for given name
    -imageFile	    image at given path
This support was added with the Cocoa-based Tk 8.5.7.

- TkAqua cursor names are interpred as follows (in order):
    - standard or platform-specific Tk cursor name (c.f. cursors.n)
    - @path to any image file readable by NSImage
    - NSImage named image name
Support for the latter two was added with the Cocoa-based Tk 8.5.7.

- The standard Tk dialog commands [tk_getOpenFile], [tk_chooseDirectory],
[tk_getSaveFile] and [tk_messageBox] all take an additional optional -command
parameter on TkAqua. If it is present, the given command prefix is evaluated at
the global level when the dialog closes, with the dialog command's result
appended (the dialog command itself returning an emtpy result). If the -parent
option is also present, the dialog is configured as a modeless (window-modal)
sheet attached to the parent window and the dialog command returns immediately.
Support for -command was added with the Cocoa-based Tk 8.5.7.

- The TkAqua-specific [tk::mac::standardAboutPanel] command brings the standard
Cocoa about panel to the front, with all its information filled in from your
application bundle files (i.e. standard about panel with no options specified).
See Apple Technote TN2179 and the AppKit documentation for -[NSApplication
orderFrontStandardAboutPanelWithOptions:] for details on the Info.plist keys and
app bundle files used by the about panel.
This support was added with the Cocoa-based Tk 8.5.7.

- TkAqua has three special menu names that give access to the standard
Application, Window and Help menus, see menu.n for details.
By default, the platform-specific standard Help menu item "YourApp Help" peforms
the default Cocoa action of showing the Help Book configured in the
application's Info.plist (or displaying an alert if no Help Book is set). This
action can be customized by defining a procedure named [tk::mac::ShowHelp], if
present, this procedure is invoked instead by the standard Help menu item.
Support for the Window menu and [tk::mac::ShowHelp] was added with the
Cocoa-based Tk 8.5.7.

- The TkAqua-specific command [tk::unsupported::MacWindowStyle style] is used to
get and set macOS-specific toplevel window class and attributes. Note that
the window class and many attributes have to be set before the window is first
mapped for the change to have any effect.
The command has the following syntax:
	tk::unsupported::MacWindowStyle style window ?class? ?attributes?
The 2 argument form returns a list of the current class and attributes for the
given window. The 3 argument form sets the class for the given window using the
default attributes for that class. The 4 argument form sets the class and the
list of attributes for the given window.
Window class names:
    document, modal, floating, utility, toolbar, simple, help, overlay
Window attribute names:
    standardDocument, standardFloating, resizable, fullZoom, horizontalZoom,
    verticalZoom, closeBox, collapseBox, toolbarButton, sideTitlebar,
    noTitleBar, unifiedTitleAndToolbar, metal, hud, noShadow, doesNotCycle,
    noActivates, hideOnSuspend, inWindowMenu, ignoreClicks, doesNotHide,
    canJoinAllSpaces, moveToActiveSpace, nonActivating

Note that not all attributes are valid for all window classes.
Support for the 3 argument form was added with the Cocoa-based Tk 8.5.7, at the
same time support for some legacy Carbon-specific classes and attributes was
removed (they are still accepted by the command but no longer have any effect).

If you want to use Remote Debugging with Xcode, you need to set the
environment variable XCNOSTDIN to 1 in the Executable editor for Wish. That will
cause us to force closing stdin & stdout.  Otherwise, given how Xcode launches
Wish remotely, they will be left open and then Wish & gdb will fight for stdin.


3. Building Tcl/Tk on macOS
------------------------------

- At least macOS 10.3 is required to build Tcl and TkX11, and macOS 10.6
is required to build TkAqua.  The XCode application provides everything
needed to build Tk, but it is not necessary to install the full XCode.
It suffices to install the Command Line Tools package, which can be done
by running the command:
xcode-selecct --install

- Tcl/Tk are most easily built as macOS frameworks via GNUmakefile in
tcl/macosx and tk/macosx (see below for details), but can also be built with the
standard unix configure and make buildsystem in tcl/unix resp. tk/unix as on any
other unix platform (indeed, the GNUmakefiles are just wrappers around the unix
buildsystem).
The macOS specific configure flags are --enable-aqua, --enable-framework and
--disable-corefoundation (which disables CF and notably reverts to the standard
select based notifier). Note that --enable-aqua is incompatible with
--disable-corefoundation (for both Tcl and Tk configure).

- It was once possible to build with the Xcode IDE via the projects in
tk/macosx, but this has not been tested recently. Take care to use the
project matching your DevTools and OS version:
	Tk.xcode: 		    for Xcode 3.1 on 10.5
	Tk.xcodeproj:		    for Xcode 3.2 on 10.6
These have the following targets:
	Tk:			    calls through to tk/macosx/GNUMakefile,
				    requires a corresponding build of the Tcl
				    target of tcl/macosx/Tcl.xcode.
	tktest:			    static build of TkAqua tktest for debugging.
	tktest-X11:		    static build of TkX11 tktest for debugging.
The following build configurations are available:
	Debug:			    debug build for the active architecture,
				    with Fix & Continue enabled.
	Debug clang:		    use clang compiler.
	Debug llvm-gcc:		    use llvm-gcc compiler.
	Debug gcc40:		    use gcc 4.0 compiler.
	DebugNoGC:		    disable Objective-C garbage collection.
	DebugNoFixAndContinue:      disable Fix & Continue.
	DebugUnthreaded:	    disable threading.
	DebugNoCF:		    disable corefoundation (X11 only).
	DebugNoCFUnthreaded:	    disable corefoundation an threading.
	DebugMemCompile:	    enable memory and bytecode debugging.
	DebugLeaks:		    define PURIFY.
	DebugGCov:		    enable generation of gcov data files.
	Debug64bit:		    configure with --enable-64bit (requires
				    building on a 64bit capable processor).
	Release:		    release build for the active architecture.
	ReleaseUniversal:	    32/64-bit universal build.
	ReleaseUniversal clang:	    use clang compiler.
	ReleaseUniversal llvm-gcc:  use llvm-gcc compiler.
	ReleaseUniversal gcc40:	    use gcc 4.0 compiler.
	ReleaseUniversal10.5SDK:    build against the 10.5 SDK (with 10.5
				    deployment target).
	Note that the non-SDK configurations have their deployment target set to
	10.5 (Tk.xcode) resp. 10.6 (Tk.xcodeproj).
The Xcode projects refer to the toplevel tcl and tk source directories via the
the TCL_SRCROOT and TK_SRCROOT user build settings, by default these are set to
the project-relative paths '../../tcl' and '../../tk', if your source
directories are named differently, e.g. '../../tcl8.6' and '../../tk8.6', you
need to manually change the TCL_SRCROOT and TK_SRCROOT settings by editing your
${USER}.pbxuser file (located inside the Tk.xcodeproj bundle directory) with a
text editor.

- To build universal binaries outside of the Xcode IDE, set CFLAGS as follows:
	export CFLAGS="-arch i386 -arch x86_64 -arch ppc"
This requires macOS 10.4 and Xcode 2.4 (or Xcode 2.2 if -arch x86_64 is
omitted, but _not_ Xcode 2.1) and will work on any architecture (on PowerPC
Tiger you need to add "-isysroot /Developer/SDKs/MacOSX10.4u.sdk").
Note that configure requires CFLAGS to contain a least one architecture that can
be run on the build machine (i.e. ppc on G3/G4, ppc or ppc64 on G5, ppc or i386
on Core and ppc, i386 or x86_64 on Core2/Xeon).
Universal builds of Tcl TEA extensions are also possible with CFLAGS set as
above, they will be [load]able by universal as well as thin binaries of Tcl.

- To enable weak-linking, set the MACOSX_DEPLOYMENT_TARGET environment variable
to the minimal OS version the binaries should be able to run on, e.g:
	export MACOSX_DEPLOYMENT_TARGET=10.6
This requires at least gcc 3.1; with gcc 4 or later, set/add to CFLAGS instead:
	export CFLAGS="-mmacosx-version-min=10.6"
Support for weak-linking was added with 8.4.14/8.5a5.

Detailed Instructions for building with macosx/GNUmakefile
----------------------------------------------------------

- Unpack the Tcl and Tk source release archives and place the tcl and tk source
trees in a common parent directory.
[ If you don't want have the two source trees in one directory, you'll need to ]
[ create the following symbolic link for the build to work as setup by default ]
[      ln -fs /path_to_tcl/build /path_to_tk/build			       ]
[ (where /path_to_{tcl,tk} is the directory containing the tcl resp. tk tree)  ]
[ or you can pass an argument of BUILD_DIR=/somewhere to the tcl and tk make.  ]

- The following instructions assume the Tcl and Tk source trees are named
"tcl${ver}" and "tk${ver}" (where ${ver} is a shell variable containing the
Tcl/Tk version number, e.g. '8.6').
Setup this shell variable as follows:
	ver="8.6"
If you are building from CVS, omit this step (CVS source tree names usually do
not contain a version number).

- Setup environment variables as desired, e.g. for a universal build on 10.5:
	CFLAGS="-arch i386 -arch x86_64 -arch ppc -mmacosx-version-min=10.5"
	export CFLAGS

- Change to the directory containing the Tcl and Tk source trees and build:
	make -C tcl${ver}/macosx
	make -C tk${ver}/macosx

- Install Tcl and Tk onto the root volume (admin password required):
	sudo make -C tcl${ver}/macosx install
	sudo make -C tk${ver}/macosx  install
if you don't have an admin password, you can install into your home directory
instead by passing an INSTALL_ROOT argument to make:
	make -C tcl${ver}/macosx install INSTALL_ROOT="${HOME}/"
	make -C tk${ver}/macosx  install INSTALL_ROOT="${HOME}/"

- The default GNUmakefile targets will build _both_ debug and optimized versions
of the Tcl and Tk frameworks with the standard convention of naming the debug
library Tcl.framework/Tcl_debug resp. Tk.framework/Tk_debug.
This allows switching to the debug libraries at runtime by setting
	export DYLD_IMAGE_SUFFIX=_debug
(c.f. man dyld for more details)

If you only want to build and install the debug or optimized build, use the
'develop' or 'deploy' target variants of the GNUmakefile, respectively.
For example, to build and install only the optimized versions:
	make -C tcl${ver}/macosx deploy
	make -C tk${ver}/macosx deploy
	sudo make -C tcl${ver}/macosx install-deploy
	sudo make -C tk${ver}/macosx  install-deploy

- The GNUmakefile can also build a version of Wish.app that has the Tcl and Tk
frameworks embedded in its application package. This allows for standalone
deployment of the application with no installation required, e.g. from read-only
media. To build & install in this manner, use the 'embedded' variants of
the GNUmakefile targets.
For example, to build a standalone 'Wish.app' in ./emb/Applications/Utilities:
	make -C tcl${ver}/macosx embedded
	make -C tk${ver}/macosx embedded
	sudo make -C tcl${ver}/macosx install-embedded INSTALL_ROOT=`pwd`/emb/
	sudo make -C tk${ver}/macosx  install-embedded INSTALL_ROOT=`pwd`/emb/
Notes:
  * if you've already built standard TclTkAqua, building embedded does not
  require any new compiling or linking, so you can skip the first two makes.
  (making relinking unnecessary was added with 8.4.2)
  * the embedded frameworks include only optimized builds and no documentation.
  * the standalone Wish has the directory Wish.app/Contents/lib in its
  auto_path. Thus you can place tcl extensions in this directory (i.e. embed
  them in the app package) and load them with [package require].

- It is possible to build Tk against an installed Tcl.framework; but you will
still need a tcl sourcetree in the location specified in TCL_SRC_DIR in
Tcl.framework/tclConfig.sh. Also, linking with Tcl.framework has to work exactly
as indicated in TCL_LIB_SPEC in Tcl.framework/tclConfig.sh.
If you used non-default install locations for Tcl.framework, specify them as
make overrides to the tk/macosx GNUmakefile, e.g.
	make -C tk${ver}/macosx \
	    TCL_FRAMEWORK_DIR=$HOME/Library/Frameworks TCLSH_DIR=$HOME/usr/bin
	sudo make -C tk${ver}/macosx install \
	    TCL_FRAMEWORK_DIR=$HOME/Library/Frameworks TCLSH_DIR=$HOME/usr/bin
The Makefile variables TCL_FRAMEWORK_DIR and TCLSH_DIR were added with Tk 8.4.3.

4. Details regarding the macOS port of Tk.
-------------------------------------------

4.1 About the event loop
~~~~~~~~~~~~~~~~~~~~~~~~

The main program in a typical OSX application looks like this (see
https://developer.apple.com/library/mac/documentation/Cocoa/\
Reference/ApplicationKit/Classes/NSApplication_Class)

    void NSApplicationMain(int argc, char *argv[]) {
        [NSApplication sharedApplication];
        [NSBundle loadNibNamed:@"myMain" owner:NSApp];
        [NSApp run];
    }
Here NSApp is a standard global variable, initialized by the OS, which
points to an object in a subclass of NSApplication (called
TKApplication in the case of the macOS port of Tk).

The [NSApp run] method implements the event loop for a typical Mac
application.  There are three key steps in the run method.  First it
calls [NSApp finishLaunching], which creates the bouncing application
icon and does other mysterious things. Second it creates an
NSAutoreleasePool.  Third, it starts an event loop which drains the
NSAutoreleasePool every time the queue is empty, and replaces the
drained pool with a new one.  This third step is essential to
preventing memory leaks, since the internal methods of Appkit objects
all assume that an autorelease pool is in scope and will be drained
when the event processing cycle ends.

The macOS Tk application does not call the [NSApp run] method at
all.  Instead it uses the event loop built in to Tk.  So the
application must take care to replicate the important features of the
method ourselves.  The way that autorelease pools are handled is
discussed in 4.2 below.  Here we discuss the event handling itself.

The Tcl event loop simply consists of repeated calls to TclDoOneEvent.
Each call to TclDoOneEvent begins by collecting all pending events from
an "event source", converting them to Tcl events and adding them
to the Tcl event queue. For macOS, the event source is the NSApp
object, which maintains an event queue even though its run method
will never be called to process them.  The NSApp provides methods for
inspecting the queue and removing events from it as well as the
[NSApp sendevent] which sends an event to all of the application's
NSWindows which can then send it to subwindows, etc.

The event collection process consists of first calling a platform
specific SetupProc and then a platform specific CheckProc.  In
the macOS port, these are named TkMacOSXEventsSetupProc and
TkMacOSXEventsCheckProc.

It is important to understand that the Apple window manager does not
have the concept of an expose event.  Their replacement for an expose
event is to have the window manager call the [NSView drawRect] method
in any situation where an expose event for that NSView would be
generated in X11.  The [NSView drawRect] method is a no-op which is
expected to be overridden by any application.  In the case of Tcl, the
replacement [NSView drawRect] method creates a Tcl expose event
for each dirty rectangle of the NSView, and then adds the expose
event to the Tcl queue.


4.2 Autorelease pools
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In order to carry out the job of managing autorelease pools, which
would normally be handled by the [NSApp run] method, a private
NSAUtoreleasePool* property is added to the TkApplication subclass of
NSApplication. The TkpInit function calls [NSApp _setup] which
initializes this property by creating an NSAutoreleasePool prior to
calling [NSApp finishLaunching].  This mimics the behavior of the
[NSApp run] method, which calls [NSApp finishLaunching] just before
starting the event loop.

Since the CheckProc function gets called for every Tk event, it is an
appropriate place to drain the main NSAutoreleasePool and replace it
with a new pool.  This is done by calling the method [NSApp
_resetAutoreleasePool], where _resetAutoreleasePool is a method which
we define for the subclass.  Unfortunately, by itself this is not
sufficient for safe memory managememt because, as was made painfully
evident with the release of OS X 10.13, it is possible for calls to
TclDoOneEvent, and hence to CheckProc, to be nested.  Draining the
autorelease pool in a nested call leads to crashes as objects in use
by the outer call can get freed by the inner call and then reused later.
One particular situation where this happens is when a modal dialogue
gets posted by a Tk Application.  To address this, the NSApp object
also implements a semaphore to prevent draining the autorelease pool
in nested calls to CheckProc.

One additional minor caveat for developers is that there are several
steps of the Tk initialization which precede the call to TkpInit.
Notably, the font package is initialized first.  Since there is no
NSAUtoreleasePool in scope prior to calling TkpInit, the functions
called in these preliminary stages need to create and drain their own
NSAutoreleasePools whenever they call methods of Appkit objects
(e.g. NSFont).

4.3 Clipping regions and "ghost windows"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Another unusual aspect of the macOS port is its use of clipping
regions.  It was part of Daniel Steffen's original design that the
TkWindowPrivate struct maintains three HIShapeRef regions, named
visRgn, aboveVisRgn and drawRgn.  These regions are used as clipping
masks whenever drawing into an NSView.  The visRgn is the bounding box
of the window with a rectangle removed for each subwindow and for each
sibling window at a higher stacking level.  The drawRgn is the
intersection of the visRgn with the clipping rectangle of the
window. (Normally, the clipping rectangle is the same as the bounding
rectangle, but drawing can be clipped to a smaller rectangle by
calling TkpClipDrawableToRect.) The aboveVisRgn is the intersection of
the window's bounding rectangle with the bounding rectangle of the
parent window.  Much of the code in tkMacOSXSubindows.c is devoted to
rebuilding these clipping regions whenever something changes in the
layout of the windows.  This turns out to be a tricky thing to do and
it is extremely prone to errors which can be difficult to trace.

It is not entirely clear what the original reason for using these
clipping regions was.  But one benefit is that if they are correctly
maintained then it allows windows to be drawn in any order.  You do
not have to draw them in the order of the window hierarchy.  Each
window can draw its entire rectangle through its own mask and never
have to worry about drawing in the wrong place.  It is likely that
the need for using clipping regions arose because, as Apple explicitly
states in the documentation for [NSView subviews],

    "The order of the subviews may be considered as being
    back-to-front, but this does not imply invalidation and drawing
    behavior."

In the early versions of the macOS port, buttons were implemented as
subviews of class TkButton.  This probably exacerbated the likelihood
that Tk windows would need to be drawn in arbitrary order.

The most obvious side effect caused by not maintaining the clipping
regions is the appearance of so-called "ghost windows".  A common
situation where these may arise is when a window containing buttons
is being scrolled.  A user may see two images of the same button on
the screen, one in the pre-scroll location and one in the post-scroll
location.

To see how these 'ghost windows' can arise, think about what happens if
the clipping regions are not maintained correctly.  A window might
have a rectangle missing from its clipping region because that
rectangle is the bounding rectangle for a subwindow, say a button.
The parent should not draw in the missing rectangle since doing so
would trash the button.  The button is responsible for drawing
there. Now imagine that the button gets moved, say by a scroll, but
the missing rectangle in the parent's clipping region does not get
moved correctly, or it gets moved later on, after the parent has
redrawn itself.  The parent would still not be allowed to draw in the
old rectangle, so the user would continue to see the image of the
button in its old location, as well as another image in the new
location.  This is a prototypical example of a "ghost window".
Anytime you see a "ghost window", you should suspect problems with the
updates to the clipping region visRgn.  It is natural to look for
timing issues, race conditions, or other "event loop problems".  But
in fact, the whole design of the code is to make those timing issues
irrelevant.  As long as the clipping regions are correctly maintained
the timing does not matter.  And if they are not correctly maintained
then you will see "ghost windows".

It is worth including a detailed description of one specific place
where the failure to correctly maintain clipping regions caused "ghost
window" artifacts that plagued the macOS port for years.  These
occurred when scrolling a Text widget which contained embedded
subwindows.  It involved some specific differences between the
low-level behavior of Apple's window manager versus those of the other
platforms, and the fix ultimately required changes in the generic Tk
implementation (documented in the comments in the DisplayText
function).

The Text widget attempts to improve perfomance when scrolling by
minimizing the number of text lines which need to be redisplayed.  It
does this by calling the platform-specific TkScrollWindow function
which uses a low-level routine to map one rectangle of the window to
another.  The TkScrollWindow function returns a damage region which is
then used by the Text widget's DisplayText function to determine which
text lines need to be redrawn.  On the unix and win platforms, this
damage region includes bounding rectangles for all embedded windows
inside the Text widget.  The way that this works is system dependent.
On unix, the low level scrolling is done by XCopyRegion, which
generates a GraphicsExpose event for each embedded window.  These
GraphicsExposed events are processsed within TkScrollWindow, using a
special handler which adds the bounding rectangle of each subwindow to
the damage region.  On the win platform the damage region is built by
the low level function ScrollWindowEx, and it also includes bounding
rectangles for all embedded windows.  This is possible because on X11
and Windows every Tk widget is also known to the window manager as a
window.  The situation is different on macOS.  The underlying object
for a top level window on macOS is the NSView.  However, Apple
explicitly warns in its documentation that performance degradation
occurs when an NSView has more than about 100 subviews.  A Text widget
with thousands of lines of text could easily contain more than 100
embedded windows.  In fact, while the original Cocoa port of Tk did
use the NSButton object, which is derived from NSView, as the basis
for its Tk Buttons, that was changed in order to improve performance.
Moreover, the low level routine used for scrolling on macOS, namely
[NSView scrollrect:by], does not provide any damage information.  So
TkScrollWindow needs to work differently on macOS.  Since it would be
inefficient to iterate through all embedded windows in a Text widget,
looking for those which meet the scrolling area, the damage region
constructed by TkScrollWindow contains only the difference between the
source and destination rectangles for the scrolling.  The embedded
windows are redrawn within the DisplayText function by some
conditional code which is only used for macOS.

