fix: window swaps flash the desktop background + redundant alpha re-stamps flicker backdrop materials

[KhangHLe]

Problem

Two independent, reproducible sources of visible flicker when tiled windows exchange positions (move --direction between two windows):

1. The desktop flashes in the vacated slot. Each window is repositioned with its own SetWindowPos call under SWP_ASYNCWINDOWPOS, so the two moves are queued to each app's own thread and land frames apart — the region one window vacates stays exposed (showing the desktop/background) until the other window's move lands. Reproducible with transparency effects disabled; most visible with two windows of the same size swapping slots.

2. Backdrop materials blink on effect passes. set_transparency re-fires SetLayeredWindowAttributes even when the value is unchanged. A redundant re-stamp isn't free: DWM re-evaluates the window's composition, which visibly flickers system-backdrop materials (Acrylic/Mica) on windows that carry them (e.g. Windows Terminal with acrylic, Firefox with Mica, Explorer). Effects passes re-apply transparency liberally, so any sync that touches effects blinks every glass window on screen.

Fix

1. Atomic repositions: plain repositions (ordinary, steady-state-visible windows — no restore, no DPI fix-up, no minimize/maximize/fullscreen special-casing, no active drag) are collected during the redraw loop and committed in a single BeginDeferWindowPos/EndDeferWindowPos transaction after it, so all rects land in the same compositor frame. A lone entry keeps the historical individual async path; anything state-special keeps the individual reposition_window path unconditionally. On any batch failure it falls back to individual repositions.

2. Idempotent alpha: set_transparency reads the current layered attributes first and returns early when the window is already layered at the target alpha.

Notes

• WindowPosBatch is a small RAII-ish wrapper in wm-platform around the defer APIs; SWP_ASYNCWINDOWPOS is stripped inside the batch (deferred transactions commit synchronously by design).
• The batch's synchronous commit matches the pre-existing trade-off documented for surrogate-adjacent repositioning; it only engages when 2+ plain repositions occur in one sync cycle.
• Developed and behavior-tested on a fork carrying additional features; this branch is the two general-purpose fixes cherry-picked onto current main and compile-verified against it.

Happy to split into two PRs if preferred.