Un-fullscreen as needed when moving fullscreen containers

This avoids a case where a fullscreen container could be moved onto a
workspace that already had its own fullscreen container, leading to
two fullscreen containers on top of each other.

src/con.c

@@ -696,6 +696,14 @@ void con_move_to_workspace(Con *con, Con *workspace, bool fix_coordinates, bool
         }
     }
 
+    /* If moving a fullscreen container and the destination already has a
+     * fullscreen window on it, un-fullscreen the target's fullscreen con. */
+    Con *fullscreen = con_get_fullscreen_con(workspace, CF_OUTPUT);
+    if (con->fullscreen_mode != CF_NONE && fullscreen != NULL) {
+        con_toggle_fullscreen(fullscreen, CF_OUTPUT);
+        fullscreen = NULL;
+    }
+
     DLOG("Re-attaching container to %p / %s\n", next, next->name);
     /* 5: re-attach the con to the parent of this focused container */
     Con *parent = con->parent;
@@ -712,8 +720,7 @@ void con_move_to_workspace(Con *con, Con *workspace, bool fix_coordinates, bool
      * invisible.
      * We don’t focus the con for i3 pseudo workspaces like __i3_scratch and
      * we don’t focus when there is a fullscreen con on that workspace. */
-    if (!con_is_internal(workspace) &&
+    if (!con_is_internal(workspace) && !fullscreen) {
-        con_get_fullscreen_con(workspace, CF_OUTPUT) == NULL) {
         /* We need to save the focused workspace on the output in case the
          * new workspace is hidden and it's necessary to immediately switch
          * back to the originally-focused workspace. */

testcases/t/100-fullscreen.t

@@ -22,7 +22,10 @@ my $i3 = i3(get_socket_path());
 my $tmp = fresh_workspace;
 
 sub fullscreen_windows {
-    scalar grep { $_->{fullscreen_mode} != 0 } @{get_ws_content($tmp)}
+    my $ws = $tmp;
+    $ws = shift if @_;
+
+    scalar grep { $_->{fullscreen_mode} != 0 } @{get_ws_content($ws)}
 }
 
 # get the output of this workspace
@@ -188,4 +191,27 @@ is($x->input_focus, $window->id, 'fullscreen window focused');
 cmd 'focus left';
 is($x->input_focus, $window->id, 'fullscreen window still focused');
 
+################################################################################
+# Verify that fullscreening a window on a second workspace and moving it onto
+# the first workspace unfullscreens the first window.
+################################################################################
+
+my $tmp2 = fresh_workspace;
+$swindow = open_window;
+
+cmd 'fullscreen';
+
+is(fullscreen_windows($tmp2), 1, 'one fullscreen window on second ws');
+
+cmd "move workspace $tmp";
+
+is(fullscreen_windows($tmp2), 0, 'no fullscreen windows on second ws');
+is(fullscreen_windows($tmp), 1, 'one fullscreen window on first ws');
+
+$swindow->fullscreen(0);
+sync_with_i3;
+
+# Verify that $swindow was the one that initially remained fullscreen.
+is(fullscreen_windows($tmp), 0, 'no fullscreen windows on first ws');
+
 done_testing;