[MG] In Tk 8.5, there's a new -fullscreen option for [wm attributes] which does this.

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toplevel .foo
wm attributes .foo -fullscreen 1
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[SPB] - 26 Apr 2012 - Moved [MG]'s comment to the top, as it is now the most relevant.

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[alove] 18 Oct 2004 -  This page illustrates a couple ways to create full-screen toplevel windows.  I wrote these for Windows XP, but the bottom one is confirmed to work for Linux too.  
If someone can tell me how to zoom a window in other OS versions, please let me know.

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package require Tcl 8.4

wm title . "Full-screen with decorations"
set a [expr {int([winfo screenwidth .] * 0.5)}]
set b [expr {int([winfo screenheight .] * 0.5)}]
wm geometry . 400x300+[expr $a - 200]+[expr $b - 150]  ;# window is centered
update idletasks 
wm attributes . -topmost no
wm state . zoomed  ;# This command for Windows only

frame .x -highlightthickness 0 -bg #c8efff
place .x -x 0 -y 0 -relwidth 1 -relheight 1

bind . <KeyPress-Escape> "wm iconify ."
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[alove] 11 Oct 2004 - This one has no decorations.

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package require Tcl 8.4

wm title . "Full-screen with NO decorations"
wm overrideredirect . yes      ;# removes window decorations
wm geometry . [winfo screenwidth .]x[winfo screenheight .]+0+0
update idletasks               ;# updates the full-screen
wm attributes . -topmost yes   ;# stays on top - needed for Linux
                               ;# on Win9x, if =no, alt-tab will malfunction
frame .x -highlightthickness 0 -bg #c8efff
place .x -x 0 -y 0 -relwidth 1 -relheight 1

button .x.min -text "Minimize" -bg #ffdbff -font "arial 10 bold" \
   -command x_iconify
place  .x.min -x [expr [winfo screenwidth  .] - 140] -y 10

button .x.end -text "Close" -bg #ffdbff -font "arial 10 bold" \
   -command "destroy ."
place  .x.end -x [expr [winfo screenwidth  .] - 60] -y 10

bind . <Expose> "wm overrideredirect . yes; focus -force ."
bind . <KeyPress-Escape> x_iconify

proc x_iconify {} {
    wm overrideredirect . no
    wm iconify .
}
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This one works on WindowsXP, probably Win9x, and according to [rdt], now also on Linux. 
Just click on the "minimize" button or the <Escape> key. 
The trick is to use <Expose> to detect that the window is being deiconified. 
The "focus force ." is necessary for Windows, otherwise the <escape> doesn't work 
until you click on the window first.

Adam Love

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[rdt] says on Linux, this works if the 'wm overrideredirect ...' comes before the 
'wm geometry ...' if the 'wm attributes ...' is commented out it gives an error.

[alove] Thanks, I've corrected the code above.  NOTE: If you set topmost=no, you will have problems with alt-tab on Windows - the icon will disappear and you can only close the program via the task manager (end task).

(by Andy M) '''window::or''' http://wiki.tcl.tk/4186 - This is a package that takes care of some nuances of going fullscreen

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[SC] Tried this on my two-screen xinerama linux setup and of course the window spans 'both' screens. A little bit of digging reveals the culprit as the X11 WidthOfScreen macro and 
a little more reveals that finding the true width of the screen isn't that hard. 
Here's a little bit of C code as memo of how it might be done:

======c
/* screen-info.c
  *
  * Compile with:
  *   cc -o screen-info screen-info.c -I/usr/X11R6/include -L/usr/X11R6/lib -lXinerama -lX11 -lXext
  *
  */

#include <stdio.h>
#include <unistd.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/extensions/Xinerama.h>

int main(int argc, char *argv[]) {
    Display *display;
    Screen *screen;
    int i;

    display = XOpenDisplay(NULL);
    if(!display) {
        printf("Couldn't open display %s\n",display);
    }

    if ( XineramaIsActive( display ) ) {
        int nscreens;
        XineramaScreenInfo * info = XineramaQueryScreens( display, &nscreens );

        printf("Xinerama is active\n");

        for( i = 0; i< nscreens; i++ ) {
            printf( "Screen %d: (%d) %d+%d+%dx%d\n", i, info[i].screen_number, 
                    info[i].x_org, info[i].y_org,
                    info[i].width, info[i].height );
        }
    } else {
        printf("Xinerama is not active\n");
    }

    screen = DefaultScreenOfDisplay(display);
    printf( "DefaultScreenOfDisplay, %d x %d\n", 
            WidthOfScreen(screen), HeightOfScreen(screen) );

    return(0);
}
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