grab - Confine pointer and keyboard events to a window sub-tree

       It took an incredibly complex and gross implementation to produce the simple grab effect described above.
       Given the current implementation, it is not safe for applications to use the Xlib grab facilities at  all
       except  through  the Tk grab procedures.  If applications try to manipulate X's grab mechanisms directly,
       things will probably break.

       If a single process is managing several different Tk applications, only one  of  those  applications  can
       have a local grab for a given display at any given time.  If the applications are in different processes,
       this restriction does not exist.

       This  command  implements  simple  pointer  and keyboard grabs for Tk.  Tk's grabs are different than the
       grabs described in the Xlib documentation.  When a grab is set for a particular window, Tk restricts  all
       pointer  events to the grab window and its descendants in Tk's window hierarchy.  Whenever the pointer is
       within the grab window's subtree, the pointer will behave exactly the same as if there had been  no  grab
       at all and all events will be reported in the normal fashion.  When the pointer is outside window's tree,
       button  presses  and releases and mouse motion events are reported to window, and window entry and window
       exit events are ignored. The grab subtree “owns” the pointer: windows outside the grab  subtree  will  be
       visible  on  the  screen  but  they  will be insensitive until the grab is released.  The tree of windows
       underneath the grab window can include top-level windows, in which case all of  those  top-level  windows
       and their descendants will continue to receive mouse events during the grab.

       Two  forms of grabs are possible:  local and global.  A local grab affects only the grabbing application:
       events will be reported to other applications as if the grab had never  occurred.   Grabs  are  local  by
       default.   A  global grab locks out all applications on the screen, so that only the given subtree of the
       grabbing application will be sensitive to pointer events (mouse button presses,  mouse  button  releases,
       pointer  motions,  window  entries,  and  window exits).  During global grabs the window manager will not
       receive pointer events either.

       During local grabs, keyboard events (key presses and key releases) are delivered as  usual:   the  window
       manager  controls  which  application receives keyboard events, and if they are sent to any window in the
       grabbing application then they are redirected to the focus window.  During a global  grab  Tk  grabs  the
       keyboard  so  that all keyboard events are always sent to the grabbing application.  The focus command is
       still used to determine which window in the application receives the keyboard events.  The keyboard  grab
       is released when the grab is released.

       On  macOS  a  global grab affects all windows created by one Tk process.  No window in that process other
       than the grab window can even be focused, hence no other window receives key or mouse  events.   A  local
       grab  on  macOS  affects  all windows created by one Tcl interpreter.  It is possible to focus any window
       belonging to the Tk process during a local grab but the grab window is the only  window  created  by  its
       interpreter  which  receives  key  or mouse events.  Windows belonging to the same process but created by
       different interpreters continue to receive key and mouse events normally.

       Grabs apply to particular displays.  If an application has windows  on  multiple  displays  then  it  can
       establish  a separate grab on each display.  The grab on a particular display affects only the windows on
       that display.  It is possible for different applications on a single display to have  simultaneous  local
       grabs, but only one application can have a global grab on a given display at once.

       The grab command can take any of the following forms:

       grab ?-global? window
              Same as grabset, described below.

       grabcurrent ?window?
              If  window  is  specified,  returns  the  name  of the current grab window in this application for
              window's display, or an empty string if there is no  such  window.   If  window  is  omitted,  the
              command  returns  a list whose elements are all of the windows grabbed by this application for all
              displays, or an empty string if the application has no grabs.

       grabreleasewindow
              Releases the grab on window if there is one, otherwise does nothing.  Returns an empty string.

       grabset ?-global? window
              Sets a grab on window.  If -global is specified then the grab is global, otherwise  it  is  local.
              If  a grab was already in effect for this application on window's display then it is automatically
              released.  If there is already a grab on window and it has  the  same  global/local  form  as  the
              requested grab, then the command does nothing.  Returns an empty string.

       grabstatuswindow
              Returns  none  if  no grab is currently set on window, local if a local grab is set on window, and
              global if a global grab is set.

       Set  a grab so that only one button may be clicked out of a group.  The other buttons are unresponsive to
       the mouse until the middle button is clicked.
              pack [button .b1 -text "Click me! #1" -command {destroy .b1}]
              pack [button .b2 -text "Click me! #2" -command {destroy .b2}]
              pack [button .b3 -text "Click me! #3" -command {destroy .b3}]
              grab .b2

       grab, keyboard events, pointer events, window

Tk                                                                                                     grab(3tk)

       grab - Confine pointer and keyboard events to a window sub-tree

busy(3tk)

grab ?-global? windowgraboption ?arg ...?
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       It is very easy to use global grabs to render a display completely unusable (e.g. by setting a grab on  a
       widget  which  does  not respond to events and not providing any mechanism for releasing the grab).  Take
       extreme care when using them!