It seems in Unity, there's always the main window for each window type (example: even when multiple Inspector window's are present in current layout, one is always the main window. Only toggling main Inspector window between normal and debug mode will result in hidden/shown anchors.

To reproduce:
1. Create a project in appropriate version
2. Create a canvas with panel, select panel to see anchor points
3. Add additional Inspector window anywhere in layout
4. In original Inspector window, change the mode to debug - anchors not shown as expected
5. Close the original Inspector window

Expected: anchors are shown again as remaining inspector is not in debug mode
Actual: anchors are still hidden as only the main Inspector window affects debug/normal modes

Reproduced in: 5.4.5p3, 5.5.4f1, 5.6.1f1, 5.6.1p4, 2017.1.0b10, 2017.2.0a4

Note: suggestion - after closing main Inspector window, other remaining Inspector window should be declared as main