Index: src/wp-admin/js/theme-plugin-editor.js
===================================================================
--- src/wp-admin/js/theme-plugin-editor.js	(revision 42771)
+++ src/wp-admin/js/theme-plugin-editor.js	(working copy)
@@ -49,6 +49,8 @@
 		component.textarea = component.form.find( '#newcontent' );
 		component.textarea.on( 'change', component.onChange );
 		component.warning = $( '.file-editor-warning' );
+		component.docsLookUpButton = component.form.find( '#docs-lookup' );
+		component.docsLookUpList = component.form.find( '#docs-list' );
 
 		if ( component.warning.length > 0 ) {
 			component.showWarning();
@@ -73,6 +75,15 @@
 			}
 			return undefined;
 		} );
+
+		component.docsLookUpList.on( 'change', function() {
+			var option = $( this ).val();
+			if ( '' === option ) {
+				component.docsLookUpButton.prop( 'disabled', true );
+			} else {
+				component.docsLookUpButton.prop( 'disabled', false );
+			}
+		} );
 	};
 
 	/**
Index: src/wp-admin/plugin-editor.php
===================================================================
--- src/wp-admin/plugin-editor.php	(revision 42771)
+++ src/wp-admin/plugin-editor.php	(working copy)
@@ -265,7 +265,7 @@
 			<input type="hidden" name="plugin" value="<?php echo esc_attr( $plugin ); ?>" />
 		</div>
 		<?php if ( ! empty( $docs_select ) ) : ?>
-		<div id="documentation" class="hide-if-no-js"><label for="docs-list"><?php _e( 'Documentation:' ); ?></label> <?php echo $docs_select; ?> <input type="button" class="button" value="<?php esc_attr_e( 'Look Up' ); ?> " onclick="if ( '' != jQuery('#docs-list').val() ) { window.open( 'https://api.wordpress.org/core/handbook/1.0/?function=' + escape( jQuery( '#docs-list' ).val() ) + '&amp;locale=<?php echo urlencode( get_user_locale() ); ?>&amp;version=<?php echo urlencode( get_bloginfo( 'version' ) ); ?>&amp;redirect=true'); }" /></div>
+		<div id="documentation" class="hide-if-no-js"><label for="docs-list"><?php _e( 'Documentation:' ); ?></label> <?php echo $docs_select; ?> <input disabled id="docs-lookup" type="button" class="button" value="<?php esc_attr_e( 'Look Up' ); ?> " onclick="if ( '' != jQuery('#docs-list').val() ) { window.open( 'https://api.wordpress.org/core/handbook/1.0/?function=' + escape( jQuery( '#docs-list' ).val() ) + '&amp;locale=<?php echo urlencode( get_user_locale() ); ?>&amp;version=<?php echo urlencode( get_bloginfo( 'version' ) ); ?>&amp;redirect=true'); }" /></div>
 		<?php endif; ?>
 <?php if ( is_writeable( $real_file ) ) : ?>
 	<div class="editor-notices">
Index: src/wp-admin/theme-editor.php
===================================================================
--- src/wp-admin/theme-editor.php	(revision 42771)
+++ src/wp-admin/theme-editor.php	(working copy)
@@ -285,7 +285,7 @@
 		<div id="documentation" class="hide-if-no-js">
 		<label for="docs-list"><?php _e( 'Documentation:' ); ?></label>
 		<?php echo $docs_select; ?>
-		<input type="button" class="button" value="<?php esc_attr_e( 'Look Up' ); ?>" onclick="if ( '' != jQuery('#docs-list').val() ) { window.open( 'https://api.wordpress.org/core/handbook/1.0/?function=' + escape( jQuery( '#docs-list' ).val() ) + '&amp;locale=<?php echo urlencode( get_user_locale() ); ?>&amp;version=<?php echo urlencode( get_bloginfo( 'version' ) ); ?>&amp;redirect=true'); }" />
+		<input disabled id="docs-lookup" type="button" class="button" value="<?php esc_attr_e( 'Look Up' ); ?>" onclick="if ( '' != jQuery('#docs-list').val() ) { window.open( 'https://api.wordpress.org/core/handbook/1.0/?function=' + escape( jQuery( '#docs-list' ).val() ) + '&amp;locale=<?php echo urlencode( get_user_locale() ); ?>&amp;version=<?php echo urlencode( get_bloginfo( 'version' ) ); ?>&amp;redirect=true'); }" />
 		</div>
 	<?php endif; ?>