case: fix button gap so that it actually surrounds the button instead of overlaps its edge

src/case.py

@@ -26,7 +26,7 @@ button_seat_margin_yz = 1.0
 button_inset_x = 0.4
 button_inset_margin_yz = 5.5
 # length of the segment we cut away entirely from the body bordering each button
-button_inset_gap = 1.2
+button_inset_gap = 1.4
 
 def _makeShell(solid, thickness: float=1, combine=False, **kwargs):
     """
@@ -122,15 +122,27 @@ def button_inset_cutaway(phone, buttons):
     return cutout
 
 def button_gap_cutaway(phone, buttons):
+    """
+    cut through the case just around the edge of the buttons, to give them a tab
+    that more easily flexes.
+    """
     buttons2d = buttons.faces("<X")
     buttons = cq.Workplane("YZ").add(buttons2d) \
         .edges().toPending().consolidateWires() \
-        .toPending().extrude(thickness)
-    keep = (buttons
-        .intersect(buttons.translate((0,  button_inset_gap, 0)))
-        .intersect(buttons.translate((0, -button_inset_gap, 0)))
+        .extrude(thickness)
+    buttons_with_buffer = cq.Workplane("YZ").add(buttons2d) \
+        .edges().toPending().offset2D(button_inset_gap) \
+        .extrude(thickness)
+    cutout = buttons_with_buffer.cut(buttons)
+    cutout_top = (cutout
+        .cut(cutout.translate((0, button_inset_gap, 0)))
+        .intersect(buttons_with_buffer.translate((0, -2*button_inset_gap, 0)))
     )
-    cutout = buttons.cut(keep)
+    cutout_bot = (cutout
+        .cut(cutout.translate((0, -button_inset_gap, 0)))
+        .intersect(buttons_with_buffer.translate((0, 2*button_inset_gap, 0)))
+    )
+    cutout = cutout_top.union(cutout_bot)
     return cutout