timeout of webrtc fallback to delay

flutter/lib/consts.dart

@@ -178,6 +178,7 @@ const String kOptionEnableIpv6Punch = "enable-ipv6-punch";
 const String kOptionAllowSyncClipboardBetweenSessions =
     "allow-sync-clipboard-between-sessions";
 const String kOptionEnableWebrtc = "enable-webrtc";
+const String kOptionRelayFallbackDelay = "relay-fallback-delay";
 const String kOptionEnableTrustedDevices = "enable-trusted-devices";
 const String kOptionShowVirtualMouse = "show-virtual-mouse";
 const String kOptionVirtualMouseScale = "virtual-mouse-scale";

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -591,13 +591,7 @@ class _GeneralState extends State<_General> {
           isServer: false,
         ),
       ],
-      if (!incomingOnly)
-        _OptionCheckBox(
-          context,
-          'Enable WebRTC P2P connection',
-          kOptionEnableWebrtc,
-          isServer: false,
-        ),
+      if (!incomingOnly) ...webrtcOptions(context),
       if (!isWeb && !incomingOnly)
         Tooltip(
           message: translate('sync-clipboard-between-sessions-tip'),
@@ -887,6 +881,85 @@ class _GeneralState extends State<_General> {
       ).marginOnly(left: _kContentHMargin);
     });
   }
+
+  // How long an already-connected relay is held back to give the direct WebRTC
+  // attempt a chance to win. It only means anything while WebRTC is on, so it
+  // follows the checkbox as an indented sub-option and is hidden outright when
+  // the box is clear — the shape `directIp` uses for its port.
+  List<Widget> webrtcOptions(BuildContext context) {
+    final stored = bind.mainGetLocalOption(key: kOptionRelayFallbackDelay);
+    final controller = TextEditingController(text: stored);
+    // What the field holds against what is saved. Apply is offered only while
+    // the two differ, so an untouched field shows no button at all, and neither
+    // does one typed back to its saved value or cleared when nothing was saved
+    // — the state an "edited" flag alone would still call dirty.
+    final typed = RxString(stored);
+    final saved = RxString(stored);
+    return [
+      _OptionCheckBox(
+        context,
+        'Enable WebRTC P2P connection',
+        kOptionEnableWebrtc,
+        isServer: false,
+        update: (_) => setState(() {}),
+      ),
+      () {
+        final enabled = mainGetLocalBoolOptionSync(kOptionEnableWebrtc);
+        final isOptFixed = isOptionFixed(kOptionRelayFallbackDelay);
+        return Offstage(
+          offstage: !enabled,
+          child: Tooltip(
+            message: translate('relay-fallback-delay-tip'),
+            child: _SubLabeledWidget(
+              context,
+              'Relay fallback delay in seconds',
+              Row(children: [
+                SizedBox(
+                  width: 95,
+                  child: TextField(
+                    controller: controller,
+                    enabled: enabled && !isOptFixed,
+                    onChanged: (v) => typed.value = v,
+                    inputFormatters: [
+                      // Seconds, at most one decimal. Clearing the field is
+                      // allowed and restores the built-in default.
+                      FilteringTextInputFormatter.allow(
+                          RegExp(r'^([0-9]|[1-9][0-9])(\.[0-9]?)?$')),
+                    ],
+                    decoration: const InputDecoration(
+                      hintText: '2.5',
+                      contentPadding:
+                          EdgeInsets.symmetric(vertical: 12, horizontal: 12),
+                    ),
+                  ).workaroundFreezeLinuxMint().marginOnly(right: 15),
+                ),
+                Obx(() => Offstage(
+                      offstage: typed.value.trim() == saved.value.trim(),
+                      child: ElevatedButton(
+                        onPressed: enabled &&
+                                !isOptFixed &&
+                                !typed.value.trim().endsWith('.') &&
+                                double.tryParse(typed.value.trim()) != 0
+                            ? () async {
+                                final v = controller.text.trim();
+                                await bind.mainSetLocalOption(
+                                    key: kOptionRelayFallbackDelay, value: v);
+                                if (controller.text != v) controller.text = v;
+                                typed.value = v;
+                                saved.value = v;
+                              }
+                            : null,
+                        child: Text(translate('Apply')),
+                      ),
+                    ))
+              ]),
+              enabled: enabled && !isOptFixed,
+            ),
+          ),
+        );
+      }(),
+    ];
+  }
 }
 
 enum _AccessMode {