Implement the configuration option floating_modifier and document it

src/config.c

@@ -215,6 +215,30 @@ void load_configuration(xcb_connection_t *conn, const char *override_configpath)
                         continue;
                 }
 
+                if (strcasecmp(key, "floating_modifier") == 0) {
+                        char *walk = value;
+                        uint32_t modifiers = 0;
+
+                        while (*walk != '\0') {
+                                /* Need to check for Mod1-5, Ctrl, Shift, Mode_switch */
+                                CHECK_MODIFIER(SHIFT);
+                                CHECK_MODIFIER(CONTROL);
+                                CHECK_MODIFIER(MODE_SWITCH);
+                                CHECK_MODIFIER(MOD1);
+                                CHECK_MODIFIER(MOD2);
+                                CHECK_MODIFIER(MOD3);
+                                CHECK_MODIFIER(MOD4);
+                                CHECK_MODIFIER(MOD5);
+
+                                /* No modifier found? Then we’re done with this step */
+                                break;
+                        }
+
+                        LOG("Floating modifiers = %d\n", modifiers);
+                        config.floating_modifier = modifiers;
+                        continue;
+                }
+
                 /* assign window class[/window title] → workspace */
                 if (strcasecmp(key, "assign") == 0) {
                         LOG("assign: \"%s\"\n", value);

src/handlers.c

@@ -302,9 +302,11 @@ int handle_button_press(void *ignored, xcb_connection_t *conn, xcb_button_press_
                 client = table_get(&by_parent, event->event);
                 border_click = true;
         }
-        /* See if this was a click with Mod1. If so, we need to move around
-         * the client if it was floating. if not, we just process as usual. */
-        if ((event->state & XCB_MOD_MASK_1) != 0) {
+        /* See if this was a click with the configured modifier. If so, we need
+         * to move around the client if it was floating. if not, we just process
+         * as usual. */
+        if (config.floating_modifier != 0 &&
+            (event->state & config.floating_modifier) != 0) {
                 if (client == NULL) {
                         LOG("Not handling, Mod1 was pressed and no client found\n");
                         return 1;