Reproduce issue #1576 as a test case (#1584)

This reproduces issue #1576 as a test case. The issue is that the frame background masks were not correctly reset; the issue was reported and fixed by @Storchschnabel.

Since the issue can be reproduced rather simply, this commit adds it as a test case.

tests/test_frame_decorations.py

@@ -1,4 +1,5 @@
 import pytest
+from conftest import PROCESS_SHUTDOWN_TIME
 
 
 @pytest.mark.parametrize("running_clients_num", [0, 1, 2])
@@ -36,3 +37,37 @@ def test_show_frame_decorations_focus(hlwm, x11):
     for v in hlwm.complete(['set', 'show_frame_decorations']):
         hlwm.attr.settings.show_frame_decorations = v
         assert len(x11.get_hlwm_frames(only_visible=True)) == expected_frame_count[v]
+
+
+def test_frame_bg_transparency_area_bug(hlwm, x11, mouse):
+    """Reproduce Issue #1576"""
+    hlwm.call(['set', 'frame_bg_transparent', 'off'])
+    hlwm.call(['set', 'always_show_frame', 'on'])
+    # put two frames side by side:
+    hlwm.call(['split', 'right', 0.5])
+    # put the mouse cursor on the right-hand frame
+    screen_size = hlwm.attr.monitors[0].geometry()
+    mouse.move_to(int(0.75 * screen_size.width), int(0.5 * screen_size.height))
+    # the focus is on the left frame. Place a client there:
+    winid, proc = hlwm.create_client()
+    hlwm.call(['resize', 'left', '+0.05'])
+    # then focus the right-hand frame
+    hlwm.call(['focus', 'right'])
+
+    # remove the client
+    hlwm.call(['close', winid])
+    proc.wait(PROCESS_SHUTDOWN_TIME)
+
+    # remove the right hand frame
+    hlwm.call(['remove'])
+
+    # now, the formerly left hand frame should span
+    # the whole screen. So lets see which window is under the
+    # mouse cursor:
+    window = x11.get_window_under_cursor()
+    # it should not be the root window
+    assert window != x11.root
+    # but the (only) frame decoration instead:
+    hlwm_frames = x11.get_hlwm_frames()
+    assert len(hlwm_frames) == 1
+    assert hlwm_frames[0] == window