Fix in tab (jupyter-widgets#1099)

* fix: do not give an error in an ipywidgets Tab

* docs: remove non-existing --overwrite flag

* chore: make this installable without needing rust

README.md

@@ -77,7 +77,7 @@ If you are using the classic Jupyter Notebook you need to install the nbextensio
 
 ```
 jupyter nbextension install --py --symlink --sys-prefix --overwrite ipyleaflet
-jupyter nbextension enable --py --sys-prefix --overwrite ipyleaflet
+jupyter nbextension enable --py --sys-prefix ipyleaflet
 ```
 
 Note for developers:

js/src/Map.js

@@ -466,6 +466,9 @@ export class LeafletMapView extends utils.LeafletDOMWidgetView {
         break;
       case 'after-show':
         this.dirty = true;
+        // If we are in a jupyter-widget tab, we get an after-show before
+        // this.displayed is resolved. In this case, obj is not created yet.
+        if(!this.obj) return;
         this.obj.invalidateSize({
           animate: false,
           pan: true,

pyproject.toml

@@ -1,3 +1,4 @@
 [build-system]
-requires = ["jupyter_packaging~=0.12", "jupyterlab>=3.0.0,==3.*", "setuptools>=40.8.0", "wheel"]
+# avoid 3.6 so we don't need the rust buildchain
+requires = ["jupyter_packaging~=0.12", "jupyterlab>=3.0.0,<3.6", "setuptools>=40.8.0", "wheel"]
 build-backend = "setuptools.build_meta"