fix: differentiate click and drag for DeviceItem, adapt tests accordingly

This fixes the blocking "QDrag.exec_()" on Linux, indeed before the
drag'n'drop operation was started with a simple click and it was
waiting for drop forever. Now there are 2 different cases, click or
drag'n'drop - the drag'n'drop test actually moves the mouse and releases
the button.

bec_widgets/widgets/services/device_browser/device_item/device_item.py

@@ -5,7 +5,7 @@ from typing import TYPE_CHECKING
 from bec_lib.logger import bec_logger
 from qtpy.QtCore import QMimeData, Qt
 from qtpy.QtGui import QDrag
-from qtpy.QtWidgets import QHBoxLayout, QLabel, QWidget
+from qtpy.QtWidgets import QApplication, QHBoxLayout, QLabel, QWidget
 
 if TYPE_CHECKING:
     from qtpy.QtGui import QMouseEvent
@@ -16,6 +16,9 @@ logger = bec_logger.logger
 class DeviceItem(QWidget):
     def __init__(self, device: str) -> None:
         super().__init__()
+
+        self._drag_pos = None
+
         self.device = device
         layout = QHBoxLayout()
         layout.setContentsMargins(10, 2, 10, 2)
@@ -32,12 +35,21 @@ class DeviceItem(QWidget):
         )
 
     def mousePressEvent(self, event: QMouseEvent) -> None:
+        super().mousePressEvent(event)
         if event.button() == Qt.LeftButton:
-            drag = QDrag(self)
-            mime_data = QMimeData()
-            mime_data.setText(self.device)
-            drag.setMimeData(mime_data)
-            drag.exec_(Qt.MoveAction)
+            self._drag_pos = event.pos()
+
+    def mouseMoveEvent(self, event: QMouseEvent) -> None:
+        if not (event.buttons() and Qt.LeftButton):
+            return
+        if (event.pos() - self._drag_pos).manhattanLength() < QApplication.startDragDistance():
+            return
+
+        drag = QDrag(self)
+        mime_data = QMimeData()
+        mime_data.setText(self.device)
+        drag.setMimeData(mime_data)
+        drag.exec_(Qt.MoveAction)
 
     def mouseDoubleClickEvent(self, event: QMouseEvent) -> None:
         logger.debug("Double Clicked")