fix: enable titlebar button scene_bufs for click and visibility

Buttons were rendered into the titlebar buffer with their own scene_buf
nodes disabled. This made them invisible to wlr_scene_node_at and
required a fragile coordinate-based hit-test (frame_buf_button_at).

Switch to rendering each button into its own frame_buf image and
enabling its scene_buf node. This makes buttons:
- Visible as independent scene nodes
- Directly hittable by wlr_scene_node_at
- Properly clickable via the normal frame_buf_at path

Update frame_buf_button_at to use a size threshold (<=30px) instead
of the disabled-node check to identify button sub-windows.

src/backend/wayland/wl_backend.c

@@ -3975,7 +3975,9 @@ frame_buf_button_at(int px, int py)
     wl_list_for_each(fb, &wl_state.frame_list, link) {
         if (!fb->parent_id)                  continue; /* top-level root */
         if (!fb->scene_buf)                  continue; /* no scene node */
-        if (fb->scene_buf->node.enabled)     continue; /* visible node — titlebar/resizebar */
+        /* Buttons are small children of the titlebar with enabled scene_bufs.
+         * Skip large nodes (titlebars, resizebars) by size threshold. */
+        if (fb->width > 30 || fb->height > 30) continue;
         if (px < fb->x || px >= fb->x + fb->width)   continue;
         if (py < fb->y || py >= fb->y + fb->height)   continue;
         /* Prefer the smallest (most specific) enclosing fb. */
@@ -7584,16 +7586,10 @@ static void wl_frame_paint_button(WCoreWindow *button, WTexture *texture,
     if (!fwin || !fwin->core)
         return;
 
-    struct wl_frame_buf *frame_fb = frame_buf_find(fwin->core->window);
-    struct wl_frame_buf *tb_fb = fwin->titlebar ? frame_buf_find(fwin->titlebar->window) : NULL;
     struct wl_frame_buf *btn_fb = frame_buf_find(button->window);
-    /* Render into titlebar buffer if available, else frame buffer */
+    if (!btn_fb || !btn_fb->image)
-    struct wl_frame_buf *target = (tb_fb && tb_fb->image) ? tb_fb : frame_fb;
-    if (!target || !target->image || !btn_fb)
         return;
 
-    int bx = btn_fb->rel_x;
-    int by = btn_fb->rel_y;
     int bw = btn_fb->width;
     int bh = btn_fb->height;
 
@@ -7613,19 +7609,36 @@ static void wl_frame_paint_button(WCoreWindow *button, WTexture *texture,
     if (!icon)
         return;
 
+    /* Clear button buffer with titlebar background */
+    struct wl_frame_buf *tb_fb = fwin->titlebar ? frame_buf_find(fwin->titlebar->window) : NULL;
+    uint32_t bg_pixel = 0xFF333333;
+    if (tb_fb && tb_fb->image) {
+        /* Sample background color from titlebar at button position */
+        uint32_t *tb_data = pixman_image_get_data(tb_fb->image);
+        int tb_stride = pixman_image_get_stride(tb_fb->image) / 4;
+        int sample_y = btn_fb->rel_y + bh / 2;
+        int sample_x = btn_fb->rel_x + bw / 2;
+        if (sample_y >= 0 && sample_y < tb_fb->height &&
+            sample_x >= 0 && sample_x < tb_fb->width)
+            bg_pixel = tb_data[sample_y * tb_stride + sample_x];
+    }
+    uint32_t *dst = pixman_image_get_data(btn_fb->image);
+    int dst_stride = pixman_image_get_stride(btn_fb->image) / 4;
+    for (int y = 0; y < bh; y++)
+        for (int x = 0; x < bw; x++)
+            dst[y * dst_stride + x] = bg_pixel;
+
     /* Blit icon centered in button area */
-    uint32_t *dst = pixman_image_get_data(target->image);
+    int ix = (bw - icon->width) / 2 + (pushed ? 1 : 0);
-    int dst_stride = pixman_image_get_stride(target->image) / 4;
+    int iy = (bh - icon->height) / 2 + (pushed ? 1 : 0);
-    int ix = bx + (bw - icon->width) / 2 + (pushed ? 1 : 0);
-    int iy = by + (bh - icon->height) / 2 + (pushed ? 1 : 0);
 
     for (int y = 0; y < icon->height; y++) {
         int dy = iy + y;
-        if (dy < 0 || dy >= target->height) continue;
+        if (dy < 0 || dy >= bh) continue;
         uint32_t *dst_row = dst + dy * dst_stride;
         for (int x = 0; x < icon->width; x++) {
             int dx = ix + x;
-            if (dx < 0 || dx >= target->width) continue;
+            if (dx < 0 || dx >= bw) continue;
             unsigned char r, g, b, a;
             if (icon->format == RRGBAFormat) {
                 int off = (y * icon->width + x) * 4;
@@ -7636,14 +7649,15 @@ static void wl_frame_paint_button(WCoreWindow *button, WTexture *texture,
                 r = icon->data[off]; g = icon->data[off+1];
                 b = icon->data[off+2]; a = 255;
             }
-            if (a == 0) continue;  /* transparent */
+            if (a == 0) continue;
             dst_row[dx] = ((uint32_t)a << 24) | ((uint32_t)r << 16) |
                           ((uint32_t)g << 8) | (uint32_t)b;
         }
     }
 
-    if (target->scene_buf)
+    /* Enable and commit the button's own scene_buf */
-        wlr_scene_buffer_set_buffer(target->scene_buf, &target->base);
+    wlr_scene_buffer_set_buffer(btn_fb->scene_buf, &btn_fb->base);
+    wlr_scene_node_set_enabled(&btn_fb->scene_buf->node, true);
 }
 
 static void wl_noop_netwm_init(WScreen *scr) { (void)scr; }