P: AI Selection Brush creates holes when painting over existing strokes

## 🐞 Bug Report: AI Selection Brush creates holes when painting over existing strokes

**Version:** Photoshop 27 (Beta)
**System:** macOS Taohe 26.0.1 (25A362)
**Hardware:** Mac Mini M2 Pro
**GPU:** Metal enabled
**Feature affected:** AI Selection Brush (Generative Fill workflow)

---

### 🧩 Description
When using the new **AI Selection Brush** to define an area for *Generative Fill*, painting over an already painted zone unexpectedly removes parts of the selection, creating semi-transparent “holes” instead of filling them solidly.

This issue does **not** occur in older stable versions of Photoshop (26.x and earlier). It appears only in the current Beta build on macOS Taohe.

---

### 🔁 Steps to Reproduce
1. Open any image.
2. Choose the **AI Selection Brush** for Generative Fill.
3. Paint an area (green overlay appears).
4. Release the mouse, then paint again over the same region.
5. Observe: instead of adding to the mask, the overlapping stroke *subtracts* or *creates transparency* within the selection.

---

### ✅ Expected Behavior
Painting over an existing AI selection should **add** to the selection (resulting in a fully opaque mask), unless the user explicitly uses Alt/Option for subtraction.

---

### ❌ Actual Behavior
- Overlapping strokes create “holes” or partial transparency inside the mask.
- The negative (Alt/Option) mode also behaves inconsistently — sometimes erasing entire zones unpredictably.
- The resulting mask appears visually corrupted and affects Generative Fill results.

---

### 🧯 Workarounds
- Use the **Lasso Tool** or **Quick Mask (Q)** to define the selection — this avoids the issue.
- Applying `Select > Modify > Expand (1 px)` before Generative Fill temporarily fixes the mask.
- Rolling back to **Photoshop 26.x** restores normal mask behavior.

---

### 🧠 Additional Info
This seems related to how the AI brush engine handles blending or vector stroke compositing under Metal on macOS Taohe.
The overlay preview shows gray (semi-transparent) values where the mask should be pure white.

---

### 📸 Screenshot
Example showing visible transparency “holes” in the AI selection overlay:

*(Replace this link with your uploaded image URL or attach it directly to the post)*

---

**Tags:** `#generativefill` `#AISelectionBrush` `#bug` `#PhotoshopBeta` `#macOS`