Show that a download is being prepared

app.css

@@ -78,3 +78,7 @@
 #atlas-database .db-actions > * { flex:0 0 auto!important; width:auto!important; min-width:0!important; padding:0 18px; }
 #atlas-database .db-toolbar fieldset .wrap { flex-wrap:nowrap; }
 #atlas-database .db-toolbar fieldset label { white-space:nowrap; }
+#db-prepare:disabled { opacity:1; cursor:progress; }
+#db-prepare:disabled::before { content:""; display:inline-block; width:11px; height:11px; margin-right:8px; vertical-align:-1px;
+  border:2px solid currentColor; border-right-color:transparent; border-radius:50%; animation:db-spin .8s linear infinite; }
+@keyframes db-spin { to { transform:rotate(360deg); } }

app.py

@@ -203,7 +203,7 @@ def build_app(catalog=None):
                         threshold = gr.Slider(0, 1, value=0.5, step=0.01, label="Threshold", min_width=200, scale=3)
                     with gr.Row(elem_classes="db-actions"):
                         view = gr.Button("Update region", variant="primary", size="sm", min_width=140, scale=0)
-                        download = gr.Button("Prepare download", size="sm", min_width=150, scale=0)
+                        download = gr.Button("Prepare download", size="sm", min_width=150, scale=0, elem_id="db-prepare")
                         file = gr.DownloadButton("Download per-base data (.parquet)", visible=False,
                                                  size="sm", min_width=240, scale=0)
                     plot = gr.Plot(show_label=False, elem_id="annotation-plot")
@@ -245,7 +245,21 @@ def build_app(catalog=None):
         view.click(update_window, region_inputs, [plot, note])
         mode.input(update_window, region_inputs, [plot, note])
         threshold.release(update_window, region_inputs, [plot, note])
-        download.click(export, selected, file)
+        # Writing a large segment takes seconds, and the only output is hidden
+        # until it is done, so the button itself has to show the work. One
+        # generator covers busy, done and failed: a chained .then does not run
+        # after an error, which left the button stuck on "Preparing".
+        def prepare(index):
+            idle = gr.Button("Prepare download", interactive=True)
+            yield gr.Button("Preparing download…", interactive=False), gr.DownloadButton(visible=False)
+            try:
+                ready = export(index)
+            except gr.Error:
+                yield idle, gr.DownloadButton(visible=False)
+                raise
+            yield idle, ready
+
+        download.click(prepare, selected, [download, file], show_progress="hidden")
     return demo