v1.6.3 — Paint fix for the back office

Fixes a rendering bug where, on some machines, back-office pages only painted after scrolling when the Dim the other items option was on. The dim effect no longer uses GPU filters (plain static opacity instead, with a lightweight hover highlight on colored items), and all will-change hints were removed. Plugin Check: clean.

This is the version to use for the WordPress.org review resubmission and for production sites running 1.4.0 or later.