fix: increase touch size for sensors, remove flicker on drag

assets/konva/app.js

@@ -351,7 +351,17 @@ function _onRoomDragEnd(room, group) {
 // ---------------------------------------------------------------------------
 
 function renderSensors() {
-  sensorsLayer.destroyChildren();
+  const activeIds = new Set(
+    sensors
+      .filter(s => s.room_id != null && s.floor_x != null)
+      .map(s => `sensor-${s.id}`)
+  );
+
+  // Remove groups for sensors that are now unplaced or gone.
+  sensorsLayer.find('Group').forEach(g => {
+    if (!activeIds.has(g.id())) g.destroy();
+  });
+
   sensors.forEach(sensor => {
     if (sensor.room_id == null || sensor.floor_x == null) return;
     const room = rooms.find(r => r.id === sensor.room_id);
@@ -361,17 +371,39 @@ function renderSensors() {
     const absY  = (room.y + sensor.floor_y) * PPM;
     const label = sensor.name ?? sensor.sensor_id;
 
+    const existing = sensorsLayer.findOne(`#sensor-${sensor.id}`);
+    if (existing) {
+      // Update in-place — avoids the destroy-rebuild flash on drag end.
+      if (!existing.isDragging()) existing.position({ x: absX, y: absY });
+      existing.draggable(mode === 'sensorMove');
+      existing.findOne('Text')?.text(label);
+      return;
+    }
+
+    _buildSensorGroup(sensor, absX, absY, label);
+  });
+  sensorsLayer.batchDraw();
+}
+
+function _buildSensorGroup(sensor, absX, absY, label) {
   const group = new Konva.Group({
     id: `sensor-${sensor.id}`,
     x: absX, y: absY,
     draggable: mode === 'sensorMove',
   });
 
+  // Visible dot — hitFunc expands the touch target without changing appearance.
   group.add(new Konva.Circle({
     radius: 8,
     fill: '#1565C0',
     stroke: '#ffffff',
     strokeWidth: 2,
+    hitFunc(ctx, shape) {
+      ctx.beginPath();
+      ctx.arc(0, 0, 20, 0, Math.PI * 2, true);
+      ctx.closePath();
+      ctx.fillStrokeShape(shape);
+    },
   }));
 
   // Highlight ring — shown by highlightSensor().
@@ -413,10 +445,13 @@ function renderSensors() {
     clearTimeout(pressTimer);
     pressTimer = null;
   });
-    group.on('dragend', () => _onSensorDragEnd(sensor, group, room));
-    sensorsLayer.add(group);
+  // Look up the current sensor data at dragend so room_id is always fresh.
+  group.on('dragend', () => {
+    const s = sensors.find(ss => ss.id === sensor.id) ?? sensor;
+    const originalRoom = rooms.find(r => r.id === s.room_id) ?? rooms[0];
+    _onSensorDragEnd(s, group, originalRoom);
   });
-  sensorsLayer.batchDraw();
+  sensorsLayer.add(group);
 }
 
 function _onSensorDragEnd(sensor, group, originalRoom) {
@@ -427,12 +462,19 @@ function _onSensorDragEnd(sensor, group, originalRoom) {
     newAbsY >= r.y && newAbsY <= r.y + r.height,
   ) ?? originalRoom;
 
+  // Optimistically update the local sensors array so any renderSensors()
+  // call that arrives before Flutter confirms the new position via loadSensors
+  // doesn't snap the group back to the old coordinates.
+  sensor.room_id = target.id;
+  sensor.floor_x = newAbsX - target.x;
+  sensor.floor_y = newAbsY - target.y;
+
   notifyFlutter({
     type: 'sensorMoved',
     id: String(sensor.id),
     roomId: target.id,
-    x: newAbsX - target.x,
-    y: newAbsY - target.y,
+    x: sensor.floor_x,
+    y: sensor.floor_y,
   });
 }