fix: prevent custom element markers from rendering upside down (#15)

internal/examples/maplibre/main.go

@@ -172,19 +172,21 @@ func main() {
 
 		// Animated container ships following waypoint routes
 		shipNames := [3]string{"MSC Adriatica", "Evergreen Harmony", "Maersk Aurora"}
-		type shipSignals struct{ lng, lat *via.Signal }
+		type shipSignals struct{ lng, lat, rot *via.Signal }
 		var ships [3]shipSignals
 
 		fleet.mu.RLock()
 		for i, s := range fleet.ships {
 			ships[i].lng = c.Signal(s.lng)
 			ships[i].lat = c.Signal(s.lat)
+			// SVG bow points right (east), so subtract 90° from the north-based heading.
+			ships[i].rot = c.Signal(s.heading() - 90)
 			m.AddMarker(fmt.Sprintf("ship-%d", i), maplibre.Marker{
-				LngSignal: ships[i].lng,
-				LatSignal: ships[i].lat,
-				Element:   shipSVG,
-				Anchor:    "center",
-				Rotation:  s.heading(),
+				LngSignal:      ships[i].lng,
+				LatSignal:      ships[i].lat,
+				RotationSignal: ships[i].rot,
+				Element:        shipSVG,
+				Anchor:         "center",
 				Popup: &maplibre.Popup{
 					Content: fmt.Sprintf("<strong>%s</strong>", shipNames[i]),
 				},
@@ -216,6 +218,7 @@ func main() {
 			for i, s := range fleet.ships {
 				ships[i].lng.SetValue(s.lng)
 				ships[i].lat.SetValue(s.lat)
+				ships[i].rot.SetValue(s.heading() - 90)
 			}
 			fleet.mu.RUnlock()
 

maplibre/js.go

@@ -180,22 +180,30 @@ func initScript(m *Map) string {
 func markerBodyJS(mapID, markerID string, mk Marker) string {
 	var b strings.Builder
 
+	// Use a wrapper div for custom elements so MapLibre rotates the div
+	// while we can independently flip the inner element to prevent inversion.
 	if mk.Element != "" {
 		b.WriteString(fmt.Sprintf(
-			`var _mkEl=document.createElement('div');_mkEl.innerHTML=%s;`,
+			`var _mkEl=document.createElement('div');`+
+				`_mkEl.style.display='inline-block';_mkEl.style.lineHeight='0';`+
+				`_mkEl.innerHTML=%s;`,
 			jsonStr(mk.Element)))
 	}
 
 	opts := "{"
 	if mk.Element != "" {
-		opts += `element:_mkEl.firstElementChild||_mkEl,`
+		opts += `element:_mkEl,`
 	} else if mk.Color != "" {
 		opts += fmt.Sprintf(`color:%s,`, jsonStr(mk.Color))
 	}
 	if mk.Anchor != "" {
 		opts += fmt.Sprintf(`anchor:%s,`, jsonStr(mk.Anchor))
 	}
-	if mk.Rotation != 0 {
+	// When both Element and RotationSignal are set, skip initial rotation
+	// in opts — we apply it post-creation with flip normalization.
+	if mk.RotationSignal != nil && mk.Element == "" {
+		opts += fmt.Sprintf(`rotation:%s,`, mk.RotationSignal.String())
+	} else if mk.RotationSignal == nil && mk.Rotation != 0 {
 		opts += fmt.Sprintf(`rotation:%s,`, formatFloat(mk.Rotation))
 	}
 	if mk.Draggable {
@@ -212,6 +220,15 @@ func markerBodyJS(mapID, markerID string, mk Marker) string {
 			opts, formatFloat(mk.LngLat.Lng), formatFloat(mk.LngLat.Lat)))
 	}
 
+	// Apply initial rotation with flip normalization for custom elements.
+	if mk.RotationSignal != nil && mk.Element != "" {
+		b.WriteString(fmt.Sprintf(
+			`var _r=%s,_f=_r>90||_r<-90;if(_f)_r=_r>0?_r-180:_r+180;`+
+				`mk.setRotation(_r);`+
+				`var _ch=_mkEl.firstElementChild;if(_ch&&_f)_ch.style.transform='scaleX(-1)';`,
+			mk.RotationSignal.String()))
+	}
+
 	if mk.Popup != nil {
 		b.WriteString(popupConstructorJS(*mk.Popup, "pk"))
 		b.WriteString(`mk.setPopup(pk);`)
@@ -250,14 +267,31 @@ func dragendHandlerJS(mapID, markerID string, mk Marker) string {
 func markerEffectExpr(mapID, markerID string, mk Marker) string {
 	// Read signals before the guard so Datastar tracks them as dependencies
 	// even when the map/marker hasn't loaded yet on first evaluation.
-	return fmt.Sprintf(
-		`var lng=$%s,lat=$%s;`+
-			`var m=window.__via_maps&&window.__via_maps[%s];`+
-			`if(m&&m._via_markers[%s]){`+
-			`m._via_markers[%s].setLngLat([lng,lat])}`,
-		mk.LngSignal.ID(), mk.LatSignal.ID(),
-		jsonStr(mapID), jsonStr(markerID), jsonStr(markerID),
-	)
+	var b strings.Builder
+	b.WriteString(fmt.Sprintf(`var lng=$%s,lat=$%s;`, mk.LngSignal.ID(), mk.LatSignal.ID()))
+	if mk.RotationSignal != nil {
+		b.WriteString(fmt.Sprintf(`var rot=$%s;`, mk.RotationSignal.ID()))
+	}
+	b.WriteString(fmt.Sprintf(
+		`var m=window.__via_maps&&window.__via_maps[%s];`+
+			`if(m&&m._via_markers[%[2]s]){`+
+			`m._via_markers[%[2]s].setLngLat([lng,lat])`,
+		jsonStr(mapID), jsonStr(markerID)))
+	if mk.RotationSignal != nil && mk.Element != "" {
+		// Normalize rotation to [-90,90] and horizontally flip the inner
+		// element when |rotation| > 90° to prevent upside-down markers.
+		b.WriteString(fmt.Sprintf(
+			`;var _mk=m._via_markers[%[1]s],_f=rot>90||rot<-90;`+
+				`if(_f)rot=rot>0?rot-180:rot+180;`+
+				`_mk.setRotation(rot);`+
+				`var _ch=_mk.getElement().firstElementChild;`+
+				`if(_ch)_ch.style.transform=_f?'scaleX(-1)':''`,
+			jsonStr(markerID)))
+	} else if mk.RotationSignal != nil {
+		b.WriteString(fmt.Sprintf(`;m._via_markers[%s].setRotation(rot)`, jsonStr(markerID)))
+	}
+	b.WriteString(`}`)
+	return b.String()
 }
 
 // addMarkerJS generates a self-contained IIFE to add a marker post-render.

maplibre/maplibre.go

@@ -122,13 +122,18 @@ func (m *Map) Element(extra ...h.H) h.H {
 		}
 	}
 
-	// Hidden inputs for signal-backed marker position writeback (drag → signal)
+	// Hidden inputs for signal-backed marker position/rotation writeback
 	for _, me := range m.markers {
 		if me.marker.LngSignal != nil && me.marker.LatSignal != nil {
 			children = append(children,
 				h.Input(h.Type("hidden"), me.marker.LngSignal.Bind()),
 				h.Input(h.Type("hidden"), me.marker.LatSignal.Bind()),
 			)
+			if me.marker.RotationSignal != nil {
+				children = append(children,
+					h.Input(h.Type("hidden"), me.marker.RotationSignal.Bind()),
+				)
+			}
 		}
 	}
 

maplibre/types.go

@@ -313,6 +313,7 @@ type Marker struct {
 	Anchor string
 
 	// Rotation is clockwise degrees. Useful for directional icons (ships, vehicles).
+	// Ignored when RotationSignal is set.
 	Rotation float64
 
 	// Signal-backed position. When set, signals drive marker position reactively.
@@ -320,6 +321,9 @@ type Marker struct {
 	// If Draggable is true, drag updates write back to these signals.
 	LngSignal *via.Signal
 	LatSignal *via.Signal
+
+	// RotationSignal drives marker rotation reactively. When set, Rotation is ignored.
+	RotationSignal *via.Signal
 }
 
 // Popup describes a map popup.