the acrylic rides move drags: only a real resize parks the glass at blur

Kyle: "when clicking and dragging the window the window gets light." It
did, by design that no longer had a reason: WM_ENTERSIZEMOVE parked the
accent at plain BLURBEHIND for EVERY modal loop, move drags included, on
the old assumption that a suspended acrylic punches through on moves the
same way it does on resizes. Plain blur ignores the gradient tint and has
none of the acrylic's material dimming — measured over a striped backdrop,
every glass surface stepped ~15% brighter (band luma 45.5 -> 52.2) for the
whole of every titlebar drag. And no compensation was possible: move loops
emit no resize-drag event, the frontend never hears about them.

Measured on this machine (research/glass-drag harness, SC_MOVE posted so
no synthetic mouse goes near the UI, GUI_INMOVESIZE as ground truth): DWM
composites the acrylic straight through a move loop. Re-asserted mid-loop
it restored rest luma EXACTLY (45.5/31.8/29.4), stationary and through 28
frames of real mouse-speed motion — no punch-through, no trailing.

So the swap is now gated on SIZING_LATCH: a titlebar move keeps the
acrylic, a latched resize parks at Blur exactly as before (DWM genuinely
suspends acrylic in live resizes — that half is load-bearing and stays).
The WM_SIZING belt-and-braces arm swaps the accent too, so a resize the
hit-test latch misses still degrades one mouse-move late instead of never.

Verified after: entering a move loop changes nothing to the decimal;
entering a resize loop still swaps (+6.7 band luma, the intended trade)
and restores on release.

src-tauri/src/thumbbar.rs

@@ -252,12 +252,20 @@ mod imp {
             SIZING_LATCH.store(true, Ordering::Relaxed);
         } else if msg == WM_ENTERSIZEMOVE {
             IN_MODAL.store(true, Ordering::Relaxed);
-            // DWM suspends the ACRYLIC accent for the whole modal loop but
-            // keeps compositing the plain BLURBEHIND one, so the window swaps
+            // DWM suspends the ACRYLIC accent inside a live RESIZE loop but
+            // keeps compositing the plain BLURBEHIND one, so a resize swaps
             // down to Blur for the gesture instead of going see-through.
-            // Every modal loop gets this — move drags included, since a
-            // suspended acrylic punches through on those exactly the same.
-            if GLASS_ON.load(Ordering::Relaxed) {
+            // A MOVE loop is different: DWM composites the acrylic straight
+            // through one — measured 2026-08-31 on this machine, stationary
+            // and at mouse-speed motion over a striped backdrop — while the
+            // Blur fallback ignores the gradient tint and skips the acrylic
+            // dimming, landing ~15% brighter over a bright backdrop. That
+            // brightness step for the whole of every titlebar drag is what
+            // "the window gets light when I drag it" was. So only a latched
+            // RESIZE parks the accent at Blur; a move keeps the acrylic. If
+            // the latch ever misses a real resize, the WM_SIZING arm below
+            // swaps the accent one mouse-move later.
+            if GLASS_ON.load(Ordering::Relaxed) && SIZING_LATCH.load(Ordering::Relaxed) {
                 let _ = apply_accent(hwnd.0, Accent::Blur);
             }
             // Arm on the LOOP, not on WM_SIZING. Press a border and hold still
@@ -279,6 +287,12 @@ mod imp {
             // cannot arm on a move. It arms one mouse-move later than the
             // latch does, which is why the latch is still the primary.
             if !IN_SIZE_DRAG.swap(true, Ordering::Relaxed) {
+                // The latch missed, so ENTERSIZEMOVE kept the acrylic; a
+                // WM_SIZING is proof this loop is a resize after all, so the
+                // accent parks at Blur here instead.
+                if GLASS_ON.load(Ordering::Relaxed) {
+                    let _ = apply_accent(hwnd.0, Accent::Blur);
+                }
                 unsafe {
                     let _ = PostMessageW(Some(hwnd), WM_RESIZE_DRAG, WPARAM(1), LPARAM(0));
                 }