window-rules: support matchOnce attribute

...allowing a rule to be applied to only the first window matching a
particular criteria. For example, the following can be used to apply a
window rule to lxqt-panel but not its configuration window with the same
app_id:

    <windowRules>
      <windowRule identifier="lxqt-panel" matchOnce="true">
        <skipTaskbar>yes</skipTaskbar>
        <action name="MoveTo" x="0" y="0" />
        <action name="ToggleAlwaysOnTop"/>
      </windowRule>
    </windowRules>

docs/labwc-config.5.scd

@@ -415,9 +415,9 @@ defined as shown below.
 </windowRules>
 ```
 
-*Actions*
+*Criteria*
 
-*<windowRules><windowRule identifier="" title="">*
+*<windowRules><windowRule identifier="" title="" matchOnce="">*
 	Define a window rule for any window which matches the criteria defined
 	by the attributes *identifier* or *title*. If both are defined, AND
 	logic is used, so both have to match.
@@ -429,6 +429,9 @@ defined as shown below.
 
 	*title* is the title of the window.
 
+	*matchOnce* can be true|false. If true, the rule will only apply to the
+	first instance of the window with the specified identifier or title.
+
 *Properties*
 
 Property values can be *yes*, *no* or *default*.

docs/rc.xml.all

@@ -391,20 +391,41 @@
   </libinput>
 
   <!--
-    Window Rules
-      - Criteria can consist of 'identifier' or 'title' or both (in which case
-        AND logic is used).
-      - 'identifier' relates to app_id for native Wayland windows and WM_CLASS
-        for XWayland clients.
-      - Matching against patterns with '*' (wildcard) and '?' (joker) is
-        supported. Pattern matching is case-insensitive.
+    # Window Rules
+    #   - Criteria can consist of 'identifier' or 'title' or both (in which case
+    #     AND logic is used).
+    #   - 'identifier' relates to app_id for native Wayland windows and WM_CLASS
+    #     for XWayland clients.
+    #   - Criteria can also contain `matchOnce="true"` meaning that the rule
+    #     must only apply to the first instance of the window with that
+    #     particular 'identifier' or 'title'.
+    #   - Matching against patterns with '*' (wildcard) and '?' (joker) is
+    #     supported. Pattern matching is case-insensitive.
 
-  <windowRules>
-    <windowRule identifier="*"><action name="Maximize"/></windowRule>
-    <windowRule identifier="foo" serverDecoration="yes"/>
-    <windowRule title="bar" serverDecoration="yes"/>
-    <windowRule identifier="baz" title="quax" serverDecoration="yes"/>
-  </windowRules>
+    <windowRules>
+      <windowRule identifier="*"><action name="Maximize"/></windowRule>
+      <windowRule identifier="foo" serverDecoration="yes"/>
+      <windowRule title="bar" serverDecoration="yes"/>
+      <windowRule identifier="baz" title="quax" serverDecoration="yes"/>
+    </windowRules>
+
+    # Example below for `lxqt-panel` and `pcmanfm-qt \-\-desktop`
+    # where 'matchOnce' is used to avoid applying rule to the panel configuration
+    # window with the same 'app_id'
+
+    <windowRules>
+      <windowRule identifier="lxqt-panel" matchOnce="true">
+        <skipTaskbar>yes</skipTaskbar>
+        <action name="MoveTo" x="0" y="0" />
+        <action name="ToggleAlwaysOnTop"/>
+      </windowRule>
+      <windowRule title="pcmanfm-desktop*">
+        <skipTaskbar>yes</skipTaskbar>
+        <skipWindowSwitcher>yes</skipWindowSwitcher>
+        <action name="MoveTo" x="0" y="0" />
+        <action name="ToggleAlwaysOnBottom"/>
+      </windowRule>
+    </windowRules>
   -->
 
 </labwc_config>

include/window-rules.h

@@ -21,6 +21,7 @@ enum property {
 struct window_rule {
 	char *identifier;
 	char *title;
+	bool match_once;
 
 	enum window_rule_event event;
 	struct wl_list actions;

src/config/rcxml.c

@@ -130,6 +130,8 @@ fill_window_rule(char *nodename, char *content)
 	} else if (!strcmp(nodename, "title")) {
 		free(current_window_rule->title);
 		current_window_rule->title = xstrdup(content);
+	} else if (!strcasecmp(nodename, "matchOnce")) {
+		set_bool(content, &current_window_rule->match_once);
 
 	/* Event */
 	} else if (!strcmp(nodename, "event")) {

src/window-rules.c

@@ -14,6 +14,33 @@
 #include "view.h"
 #include "window-rules.h"
 
+static bool
+other_instances_exist(struct view *self, const char *id, const char *title)
+{
+	struct wl_list *views = &self->server->views;
+	const char *prop = NULL;
+	struct view *view;
+
+	wl_list_for_each(view, views, link) {
+		if (view == self) {
+			continue;
+		}
+		if (id) {
+			prop = view_get_string_prop(view, "app_id");
+			if (prop && !strcmp(prop, id)) {
+				return true;
+			}
+		}
+		if (title) {
+			prop = view_get_string_prop(view, "title");
+			if (prop && !strcmp(prop, title)) {
+				return true;
+			}
+		}
+	}
+	return false;
+}
+
 /* Try to match against identifier AND title (if set) */
 static bool
 view_matches_criteria(struct window_rule *rule, struct view *view)
@@ -21,6 +48,10 @@ view_matches_criteria(struct window_rule *rule, struct view *view)
 	const char *id = view_get_string_prop(view, "app_id");
 	const char *title = view_get_string_prop(view, "title");
 
+	if (rule->match_once && other_instances_exist(view, id, title)) {
+		return false;
+	}
+
 	if (rule->identifier && rule->title) {
 		if (!id || !title) {
 			return false;