Assign drags to first-empty pane and ensure mpv ready before load

cmd/gtkplayer/main.go

@@ -42,6 +42,8 @@ type pane struct {
 	path string
 }
 
+func (p *pane) hasVideo() bool { return p.path != "" }
+
 func main() {
 	gtk.Init(nil)
 
@@ -68,8 +70,7 @@ func main() {
 	applyCSS()
 	preferDark()
 
-	setupDragDest(left, win)
-	setupDragDest(right, win)
+	setupDragDest(left, right, win)
 
 	win.Connect("destroy", func() {
 		if left.mpv != nil {
@@ -213,7 +214,7 @@ func chooseAndLoad(win *gtk.Window, p *pane) {
 }
 
 func loadIntoPane(p *pane, filename string) {
-	if p.mpv == nil {
+	if !ensurePaneReady(p) {
 		return
 	}
 	p.path = filename
@@ -306,14 +307,59 @@ func setupDragDest(p *pane, win *gtk.Window) {
 			if ln == "" {
 				continue
 			}
-			if path := uriToPath(ln); path != "" {
-				loadIntoPane(p, path)
-				break
-			}
+			assignPathToPane(ln, p, nil)
+			break
 		}
 	})
 }
 
+// decide which pane to load based on availability: left prefers first, right second.
+func assignPathToPane(uri string, left, right *pane) {
+	path := uriToPath(uri)
+	if path == "" {
+		return
+	}
+	if left != nil && !left.hasVideo() {
+		loadIntoPane(left, path)
+		return
+	}
+	if right != nil && !right.hasVideo() {
+		loadIntoPane(right, path)
+		return
+	}
+	// default: replace left
+	if left != nil {
+		loadIntoPane(left, path)
+	}
+}
+
+func ensurePaneReady(p *pane) bool {
+	if p == nil {
+		return false
+	}
+	if p.mpv != nil {
+		return true
+	}
+	mpv, err := mpvembed.New()
+	if err != nil {
+		log.Printf("mpv create: %v", err)
+		return false
+	}
+	// Bind window if realized
+	if w, err := p.area.GetWindow(); err == nil && w != nil {
+		if xid := getWindowID(w); xid != 0 {
+			_ = mpv.SetWID(xid)
+		}
+	}
+	_ = mpv.SetOptionString("pause", "yes")
+	if err := mpv.Initialize(); err != nil {
+		log.Printf("mpv init: %v", err)
+		return false
+	}
+	p.mpv = mpv
+	return true
+}
+
 func uriToPath(u string) string {
 	if u == "" {
 		return ""