RF: set labelHeight to match tick height by default

Author: peircej

psychopy/visual/slider.py

@@ -163,7 +163,7 @@ def __init__(self,
         self.tickLines = []
         self.tickLocs = None
         self.labelLocs = None
-        self.labelHeight = labelHeight
+        self.labelHeight = labelHeight or min(self._size)
         self._lineAspectRatio = 0.01
         self._updateMarkerPos = True
         self._dragging = False