fix(bar): clear blur region when the bar is hidden (auto-hide) (#2658)

When a bar with background transparency + blur uses auto-hide, the
ext-background-effect-v1 blur region was only slid off-surface via the
reveal Translate, but the region object stayed non-empty.

Hyprland gates layer-surface blur on `!m_blurRegion.empty()`, so the
non-empty region keeps blur enabled; the renderer then intersects the
off-surface region with the surface box, the clip degenerates to empty,
and an empty clip is treated as "unclipped" — so the whole bar surface
box gets blurred, leaving a blurred strip where the hidden bar would be.
(niri clips correctly, so it never showed there.)

Gate the published blur region on `barRevealed`: tear it down (null)
whenever the bar is not currently shown, so the region is genuinely
empty and the compositor disables the effect. Fixes #2656.

quickshell/Modules/DankBar/DankBarWindow.qml

@@ -150,6 +150,9 @@ PanelWindow {
         function onUsesFrameBarChromeChanged() {
             _blurRebuildTimer.restart();
         }
+        function onBarRevealedChanged() {
+            _blurRebuildTimer.restart();
+        }
     }
 
     Component {
@@ -176,6 +179,13 @@ PanelWindow {
             teardown();
             if (!BlurService.enabled || !BlurService.available)
                 return;
+            // When the bar is hidden (auto-hide, or config not visible) keep the blur
+            // region empty rather than sliding it off-surface. Some compositors (Hyprland)
+            // gate blur on a non-empty region and then blur the whole surface box when the
+            // clip degenerates to empty, leaving the bar strip blurred while the bar is
+            // hidden (issue #2656). A null region disables the effect cleanly.
+            if (!barWindow.barRevealed)
+                return;
             // In frame mode, FrameWindow owns the blur region for the entire screen edge
             // (including the bar area). The bar must not set its own competing blur region
             // so that frameBlurEnabled acts as the single control for all blur in frame mode.