Clear stale hover on capture-loss and Alt-delete; derive gutter test via chromeRects

src/shell/instrument/editor_input.cpp

@@ -82,7 +82,10 @@ void ReaSamplerEditor::onMouseUp(int x, int y) {
     // hover_ is deliberately not re-resolved during a drag (see resolveHover's caller), so it
     // still names wherever the drag started. Re-resolve now against the release position, for
     // every drag kind — otherwise the next paint latches a stale hover (wrong note name/tooltip,
-    // wrong control outline) until the next WM_MOUSEMOVE.
+    // wrong control outline) until the next WM_MOUSEMOVE. This resolve runs before the release
+    // branches below can delete or relocate the hovered element; a future branch that does so
+    // must clear hover_ itself afterwards (as the curve drag-off delete does below) rather than
+    // rely on this resolve, which reflects pre-mutation state.
     resolveHover(x, y);
     // A scrollbar drag is transient UI (no parameter change), and the processor-side knobs
     // (the preview-velocity -2 sentinel, voice count, master gain) are per-instance settings