diff --git a/vasl_templates/tools/webdriver_stress_test.py b/vasl_templates/tools/webdriver_stress_test.py
new file mode 100755
index 0000000..976b6b8
--- /dev/null
+++ b/vasl_templates/tools/webdriver_stress_test.py
@@ -0,0 +1,256 @@
+#!/usr/bin/env python3
+""" Stress-test the shared WebDriver. """
+
+import os
+import threading
+import signal
+import http.client
+import time
+import datetime
+import base64
+import random
+import json
+import logging
+from collections import defaultdict
+
+from selenium.webdriver.common.action_chains import ActionChains
+from selenium.webdriver.common.keys import Keys
+import click
+
+from vasl_templates.webapp.webdriver import WebDriver
+from vasl_templates.webapp.tests.test_scenario_persistence import load_scenario
+from vasl_templates.webapp.tests.utils import wait_for, find_child, find_snippet_buttons, \
+    select_tab, select_menu_option, click_dialog_button, set_stored_msg, get_stored_msg
+
+shutdown_event = threading.Event()
+thread_count = None
+
+stats_lock = threading.Lock()
+stats = defaultdict( lambda: [0,0] ) # nb: [ #runs, total elapsed time ]
+
+# ---------------------------------------------------------------------
+
+@click.command()
+@click.option( "--server-url", default="http://localhost:5010", help="Webapp server URL." )
+@click.option( "--snippet-images", default=1, help="Number of 'snippet image' threads to run." )
+@click.option( "--update-vsav", default=1, help="Number of 'update VSAV' threads to run." )
+@click.option( "--vsav","vsav_fname", help="VASL scenario file (.vsav) to be updated." )
+def main( server_url, snippet_images, update_vsav, vsav_fname ):
+    """Stress-test the shared WebDriver."""
+
+    # initialize
+    logging.disable( logging.CRITICAL )
+
+    # read the VASL scenario file
+    vsav_data = None
+    if update_vsav > 0:
+        vsav_data = open( vsav_fname, "rb" ).read()
+
+    # prepare the test threads
+    threads = []
+    for i in range(0,snippet_images):
+        threads.append( threading.Thread(
+            target = snippet_images_thread,
+            name = "snippet-images/{:02d}".format( 1+i ),
+            args = ( server_url, )
+        ) )
+    for i in range(0,update_vsav):
+        threads.append( threading.Thread(
+            target = update_vsav_thread,
+            name = "update-vsav/{:02d}".format( 1+i ),
+            args = ( server_url, vsav_fname, vsav_data )
+        ) )
+
+    # launch the test threads
+    start_time = time.time()
+    global thread_count
+    thread_count = len(threads)
+    for thread in threads:
+        thread.start()
+
+    # wait for Ctrl-C
+    def on_sigint( signum, stack ): #pylint: disable=missing-docstring,unused-argument
+        print( "\n*** SIGINT received ***\n" )
+        shutdown_event.set()
+    signal.signal( signal.SIGINT, on_sigint )
+    while not shutdown_event.is_set():
+        time.sleep( 1 )
+
+    # wait for the test threads to shutdown
+    for thread in threads:
+        print( "Waiting for thread to finish:", thread )
+        thread.join()
+    elapsed_time = time.time() - start_time
+    print()
+
+    # output stats
+    print( "=== STATS ===")
+    print()
+    print( "Total run time: {}".format( datetime.timedelta( seconds=int(elapsed_time) ) ) )
+    for key,val in stats.items():
+        print( "- {:<14} {}".format( key+":", val[0] ), end="" )
+        if val[0] > 0:
+            print( " (avg={:.3f}s)".format( float(val[1])/val[0] ) )
+        else:
+            print()
+
+# ---------------------------------------------------------------------
+
+def snippet_images_thread( server_url ):
+    """Test generating snippet images."""
+
+    with WebDriver() as webdriver:
+
+        # initialize
+        webdriver = webdriver.driver
+        init_webapp( webdriver, server_url,
+             [ "snippet_image_persistence", "scenario_persistence" ]
+        )
+
+        # load a scenario (so that we get some sortable's)
+        scenario_data = {
+            "SCENARIO_NOTES": [ { "caption": "Scenario note #1"  } ],
+            "OB_SETUPS_1": [ { "caption": "OB setup note #1" } ],
+            "OB_NOTES_1": [ { "caption": "OB note #1" } ],
+            "OB_SETUPS_2": [ { "caption": "OB setup note #2" } ],
+            "OB_NOTES_2": [ { "caption": "OB note #2" } ],
+        }
+        load_scenario( scenario_data, webdriver )
+
+        # locate all the "generate snippet" buttons
+        snippet_btns = find_snippet_buttons( webdriver )
+        tab_ids = list( snippet_btns.keys() )
+
+        while not shutdown_event.is_set():
+
+            try:
+                # clear the return buffer
+                ret_buffer = find_child( "#_snippet-image-persistence_", webdriver )
+                assert ret_buffer.tag_name == "textarea"
+                webdriver.execute_script( "arguments[0].value = arguments[1]", ret_buffer, "" )
+
+                # generate a snippet
+                tab_id = random.choice( tab_ids )
+                btn = random.choice( snippet_btns[ tab_id ] )
+                log( "Getting snippet image: {}", btn.get_attribute("data-id") )
+                select_tab( tab_id, webdriver )
+                start_time = time.time()
+                ActionChains( webdriver ) \
+                    .key_down( Keys.SHIFT ) \
+                    .click( btn ) \
+                    .key_up( Keys.SHIFT ) \
+                    .perform()
+
+                # wait for the snippet image to be generated
+                wait_for( 10*thread_count, lambda: ret_buffer.get_attribute( "value" ) )
+                _, img_data = ret_buffer.get_attribute( "value" ).split( "|", 1 )
+                elapsed_time = time.time() - start_time
+
+                # update the stats
+                with stats_lock:
+                    stats["snippet image"][0] += 1
+                    stats["snippet image"][1] += elapsed_time
+
+                # FUDGE! Generating the snippet image for a sortable entry is sometimes interpreted as
+                # a request to edit the entry (Selenium problem?) - we dismiss the dialog here and continue.
+                dlg = find_child( ".ui-dialog", webdriver )
+                if dlg and dlg.is_displayed():
+                    click_dialog_button( "Cancel", webdriver )
+
+            except ( ConnectionRefusedError, ConnectionResetError, http.client.RemoteDisconnected ):
+                if shutdown_event.is_set():
+                    break
+                raise
+
+            # check the generated snippet
+            img_data = base64.b64decode( img_data )
+            log( "Received snippet image: #bytes={}", len(img_data) )
+            assert img_data[:6] == b"\x89PNG\r\n"
+
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+def update_vsav_thread( server_url, vsav_fname, vsav_data ):
+    """Test updating VASL scenario files."""
+
+    # initialize
+    vsav_data_b64 = base64.b64encode( vsav_data ).decode( "utf-8" )
+
+    with WebDriver() as webdriver:
+
+        # initialize
+        webdriver = webdriver.driver
+        init_webapp( webdriver, server_url,
+             [ "vsav_persistence", "scenario_persistence" ]
+        )
+
+        # load a test scenario
+        fname = os.path.join( os.path.split(__file__)[0], "../webapp/tests/fixtures/update-vsav/full.json" )
+        saved_scenario = json.load( open( fname, "r" ) )
+        load_scenario( saved_scenario, webdriver )
+
+        while not shutdown_event.is_set():
+
+            try:
+
+                # send the VSAV data to the front-end to be updated
+                log( "Updating VSAV: {}", vsav_fname )
+                set_stored_msg( "_vsav-persistence_", vsav_data_b64, webdriver )
+                select_menu_option( "update_vsav", webdriver )
+                start_time = time.time()
+
+                # wait for the front-end to receive the data
+                wait_for( 2*thread_count,
+                    lambda: get_stored_msg( "_vsav-persistence_", webdriver ) == ""
+                )
+
+                # wait for the updated data to arrive
+                wait_for( 60*thread_count,
+                    lambda: get_stored_msg( "_vsav-persistence_", webdriver ) != ""
+                )
+                elapsed_time = time.time() - start_time
+
+                # get the updated VSAV data
+                updated_vsav_data = get_stored_msg( "_vsav-persistence_", webdriver )
+                if updated_vsav_data.startswith( "ERROR: " ):
+                    raise RuntimeError( updated_vsav_data )
+                updated_vsav_data = base64.b64decode( updated_vsav_data )
+
+                # check the updated VSAV
+                log( "Received updated VSAV data: #bytes={}", len(updated_vsav_data) )
+                assert updated_vsav_data[:2] == b"PK"
+
+                # update the stats
+                with stats_lock:
+                    stats["update vsav"][0] += 1
+                    stats["update vsav"][1] += elapsed_time
+
+            except (ConnectionRefusedError, ConnectionResetError, http.client.RemoteDisconnected):
+                if shutdown_event.is_set():
+                    break
+                raise
+
+# ---------------------------------------------------------------------
+
+def log( fmt, *args, **kwargs ):
+    """Log a message."""
+    now = time.time()
+    msec = now - int(now)
+    now = "{}.{:03d}".format( time.strftime("%H:%M:%S",time.localtime(now)), int(msec*1000) )
+    msg = fmt.format( *args, **kwargs )
+    msg = "{} | {:17} | {}".format( now, threading.current_thread().name, msg )
+    print( msg )
+
+# ---------------------------------------------------------------------
+
+def init_webapp( webdriver, server_url, options ):
+    """Initialize the webapp."""
+    log( "Initializing the webapp." )
+    url = server_url + "?" + "&".join( "{}=1".format(opt) for opt in options )
+    url += "&store_msgs=1" # nb: stop notification balloons from building up
+    webdriver.get( url )
+    wait_for( 5, lambda: find_child("#_page-loaded_",webdriver) is not None )
+
+# ---------------------------------------------------------------------
+
+if __name__ == "__main__":
+    main() #pylint: disable=no-value-for-parameter
diff --git a/vasl_templates/webapp/__init__.py b/vasl_templates/webapp/__init__.py
index 1ab2a5d..4ef3a8d 100644
--- a/vasl_templates/webapp/__init__.py
+++ b/vasl_templates/webapp/__init__.py
@@ -2,6 +2,7 @@
 
 import sys
 import os
+import signal
 import configparser
 import logging
 import logging.config
@@ -28,6 +29,16 @@ def load_debug_config( fname ):
 
 # ---------------------------------------------------------------------
 
+cleanup_handlers = []
+
+def on_sigint( signum, stack ): #pylint: disable=unused-argument
+    """Clean up after a SIGINT."""
+    for handler in cleanup_handlers:
+        handler()
+    raise SystemExit()
+
+# ---------------------------------------------------------------------
+
 # initialize Flask
 app = Flask( __name__ )
 
@@ -65,6 +76,9 @@ if app.config.get( "ENABLE_REMOTE_TEST_CONTROL" ):
     print( "*** WARNING: Remote test control enabled! ***" )
     import vasl_templates.webapp.testing #pylint: disable=cyclic-import
 
+# install our signal handler (must be done in the main thread)
+signal.signal( signal.SIGINT, on_sigint )
+
 # ---------------------------------------------------------------------
 
 @app.context_processor
diff --git a/vasl_templates/webapp/snippets.py b/vasl_templates/webapp/snippets.py
index 2376050..e98bdfb 100644
--- a/vasl_templates/webapp/snippets.py
+++ b/vasl_templates/webapp/snippets.py
@@ -120,7 +120,7 @@ def make_snippet_image():
     # generate an image for the snippet
     snippet = request.data.decode( "utf-8" )
     try:
-        with WebDriver() as webdriver:
+        with WebDriver.get_instance() as webdriver:
             img = webdriver.get_snippet_screenshot( None, snippet )
     except SimpleError as ex:
         return "ERROR: {}".format( ex )
diff --git a/vasl_templates/webapp/static/snippets.js b/vasl_templates/webapp/static/snippets.js
index e890d84..683a581 100644
--- a/vasl_templates/webapp/static/snippets.js
+++ b/vasl_templates/webapp/static/snippets.js
@@ -31,21 +31,26 @@ function generate_snippet( $btn, evt, extra_params )
     // check if the user is requesting the snippet as an image
     if ( evt.shiftKey ) {
         // yup - send the snippet to the backend to generate the image
-        var $dlg = $( "#make-snippet-image" ).dialog( {
-            dialogClass: "make-snippet-image",
-            modal: true,
-            width: 300,
-            height: 60,
-            resizable: false,
-            closeOnEscape: false,
-        } ) ;
+        var $dlg = null ;
+        var timeout_id = setTimeout( function() {
+            $dlg = $( "#make-snippet-image" ).dialog( {
+                dialogClass: "make-snippet-image",
+                modal: true,
+                width: 300,
+                height: 60,
+                resizable: false,
+                closeOnEscape: false,
+            } ) ;
+        }, 1*1000 ) ;
         $.ajax( {
             url: gMakeSnippetImageUrl,
             type: "POST",
             data: snippet.content,
             contentType: "text/html",
         } ).done( function( resp ) {
-            $dlg.dialog( "close" ) ;
+            clearTimeout( timeout_id ) ;
+            if ( $dlg )
+                $dlg.dialog( "close" ) ;
             if ( resp.substr( 0, 6 ) === "ERROR:" ) {
                 showErrorMsg( resp.substr(7) ) ;
                 return ;
@@ -68,7 +73,9 @@ function generate_snippet( $btn, evt, extra_params )
                 download( atob(resp), _make_snippet_image_filename(snippet), "image/png" ) ;
             }
         } ).fail( function( xhr, status, errorMsg ) {
-            $dlg.dialog( "close" ) ;
+            clearTimeout( timeout_id ) ;
+            if ( $dlg )
+                $dlg.dialog( "close" ) ;
             showErrorMsg( "Can't get the snippet image:<div class='pre'>" + escapeHTML(errorMsg) + "</div>" ) ;
         } ) ;
         return ;
diff --git a/vasl_templates/webapp/tests/test_scenario_persistence.py b/vasl_templates/webapp/tests/test_scenario_persistence.py
index bdb5688..a98f661 100644
--- a/vasl_templates/webapp/tests/test_scenario_persistence.py
+++ b/vasl_templates/webapp/tests/test_scenario_persistence.py
@@ -287,12 +287,14 @@ def test_unknown_vo( webapp, webdriver ):
 
 # ---------------------------------------------------------------------
 
-def load_scenario( scenario ):
+def load_scenario( scenario, webdriver=None ):
     """Load a scenario into the UI."""
-    set_stored_msg( "_scenario-persistence_", json.dumps(scenario) )
-    _ = set_stored_msg_marker( "_last-info_" )
-    select_menu_option( "load_scenario" )
-    wait_for( 2, lambda: get_stored_msg("_last-info_") == "The scenario was loaded." )
+    set_stored_msg( "_scenario-persistence_", json.dumps(scenario), webdriver )
+    _ = set_stored_msg_marker( "_last-info_", webdriver )
+    select_menu_option( "load_scenario", webdriver )
+    wait_for( 2,
+        lambda: get_stored_msg( "_last-info_", webdriver ) == "The scenario was loaded."
+    )
 
 def save_scenario():
     """Save the scenario."""
diff --git a/vasl_templates/webapp/tests/test_snippets.py b/vasl_templates/webapp/tests/test_snippets.py
index f6f31e4..69f3a5b 100644
--- a/vasl_templates/webapp/tests/test_snippets.py
+++ b/vasl_templates/webapp/tests/test_snippets.py
@@ -4,7 +4,7 @@ from selenium.webdriver.common.action_chains import ActionChains
 from selenium.webdriver.common.keys import Keys
 
 from vasl_templates.webapp.tests.utils import \
-    init_webapp, select_tab, select_tab_for_elem, set_template_params, wait_for, wait_for_clipboard, \
+    init_webapp, select_tab, find_snippet_buttons, set_template_params, wait_for, wait_for_clipboard, \
     get_stored_msg, set_stored_msg_marker, find_child, find_children, adjust_html, \
     for_each_template, add_simple_note, edit_simple_note, \
     get_sortable_entry_count, generate_sortable_entry_snippet, drag_sortable_entry_to_trash, \
@@ -33,12 +33,6 @@ def test_snippet_ids( webapp, webdriver ):
 
     def check_snippet( btn ):
         """Generate a snippet and check that it has an ID."""
-        select_tab_for_elem( btn )
-        if not btn.is_displayed():
-            # FUDGE! All nationality-specific buttons are created on each OB tab, and the ones not relevant
-            # to each player are just hidden. This is not real good since we have multiple elements with the same ID :-/
-            # but we work around this by checking if the button is visible. Sigh...
-            return
         btn.click()
         wait_for_clipboard( 2, "<!-- vasl-templates:id ", contains=True )
 
@@ -49,10 +43,11 @@ def test_snippet_ids( webapp, webdriver ):
         set_scenario_date( scenario_date )
 
         # check each snippet
-        for btn in find_children( "button.generate" ):
-            check_snippet( btn )
-        for btn in find_children( "img.snippet" ):
-            check_snippet( btn )
+        snippet_btns = find_snippet_buttons()
+        for tab_id,btns in snippet_btns.items():
+            select_tab( tab_id )
+            for btn in btns:
+                check_snippet( btn )
 
     # test snippets with German/Russian
     do_test( "" )
diff --git a/vasl_templates/webapp/tests/utils.py b/vasl_templates/webapp/tests/utils.py
index 65a38db..5463e3a 100644
--- a/vasl_templates/webapp/tests/utils.py
+++ b/vasl_templates/webapp/tests/utils.py
@@ -6,6 +6,7 @@ import json
 import time
 import re
 import uuid
+from collections import defaultdict
 
 import pytest
 from PyQt5.QtWidgets import QApplication
@@ -115,31 +116,42 @@ def for_each_template( func ): #pylint: disable=too-many-branches
 
 # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
-def select_tab( tab_id ):
+def select_tab( tab_id, webdriver=None ):
     """Select a tab in the main page."""
-    elem = find_child( "#tabs .ui-tabs-nav a[href='#tabs-{}']".format( tab_id ) )
+    if not webdriver:
+        webdriver = _webdriver
+    elem = find_child( "#tabs .ui-tabs-nav a[href='#tabs-{}']".format( tab_id ), webdriver )
     elem.click()
 
 def select_tab_for_elem( elem ):
     """Select the tab that contains the specified element."""
-    while True:
+    select_tab( get_tab_for_elem( elem ) )
+
+def get_tab_for_elem( elem ):
+    """Identify the tab that contains the specified element."""
+    while elem.tag_name not in ("html","body"):
         elem = elem.find_element_by_xpath( ".." )
         if elem.tag_name == "div":
             div_id = elem.get_attribute( "id" )
             if div_id.startswith( "tabs-" ):
-                select_tab( div_id[5:] )
-                break
+                return div_id[5:]
+    return None
 
-def select_menu_option( menu_id ):
+def select_menu_option( menu_id, webdriver=None ):
     """Select a menu option."""
-    elem = find_child( "#menu" )
+    if not webdriver:
+        webdriver = _webdriver
+    elem = find_child( "#menu", webdriver )
     elem.click()
-    elem = wait_for_elem( 2, "a.PopMenu-Link[data-name='{}']".format( menu_id ) )
+    elem = wait_for_elem( 2, "a.PopMenu-Link[data-name='{}']".format( menu_id ), webdriver )
     elem.click()
-    wait_for( 2, lambda: find_child("#menu .PopMenu-Container") is None ) # nb: wait for the menu to go away
-    if pytest.config.option.webdriver == "chrome": #pylint: disable=no-member
-        # FUDGE! Work-around weird "is not clickable" errors because the PopMenu is still around :-/
-        time.sleep( 0.25 )
+    wait_for( 2, lambda: find_child("#menu .PopMenu-Container",webdriver) is None ) # nb: wait for the menu to go away
+    try:
+        if pytest.config.option.webdriver == "chrome": #pylint: disable=no-member
+            # FUDGE! Work-around weird "is not clickable" errors because the PopMenu is still around :-/
+            time.sleep( 0.25 )
+    except AttributeError:
+        pass
 
 def new_scenario():
     """Reset the scenario."""
@@ -323,24 +335,28 @@ def find_sortable_helper( sortable, tag ):
 
 # ---------------------------------------------------------------------
 
-def get_stored_msg( msg_type ):
+def get_stored_msg( msg_type, webdriver=None ):
     """Get a message stored for us by the front-end."""
-    elem = find_child( "#" + msg_type )
+    if not webdriver:
+        webdriver = _webdriver
+    elem = find_child( "#" + msg_type, webdriver )
     assert elem.tag_name == "textarea"
     return elem.get_attribute( "value" )
 
-def set_stored_msg( msg_type, val ):
+def set_stored_msg( msg_type, val, webdriver=None ):
     """Set a message for the front-end."""
-    elem = find_child( "#" + msg_type )
+    if not webdriver:
+        webdriver = _webdriver
+    elem = find_child( "#" + msg_type, webdriver )
     assert elem.tag_name == "textarea"
-    _webdriver.execute_script( "arguments[0].value = arguments[1]", elem, val )
+    webdriver.execute_script( "arguments[0].value = arguments[1]", elem, val )
 
-def set_stored_msg_marker( msg_type ):
+def set_stored_msg_marker( msg_type, webdriver=None ):
     """Store marker text in the message buffer (so we can tell if the front-end changes it)."""
     # NOTE: Care should taken when using this function with "_clipboard_",
     # since the tests might be using the real clipboard!
     marker = "marker:{}:{}".format( msg_type, uuid.uuid4() )
-    set_stored_msg( msg_type, marker )
+    set_stored_msg( msg_type, marker, webdriver )
     return marker
 
 # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
@@ -362,6 +378,29 @@ def find_children( sel, parent=None ):
 
 # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+def find_snippet_buttons( webdriver=None ):
+    """Find all "generate snippet" buttons.
+
+    NOTE: We only return the 1st snippet button in the "extras" tab.
+    """
+    snippet_btns = defaultdict( list )
+    # find all normal "generate snippet" buttons
+    for btn in find_children( "button.generate", webdriver ):
+        snippet_btns[ get_tab_for_elem(btn) ].append( btn )
+    # find "generate snippet" buttons in sortable lists
+    for btn in find_children( "ul.sortable img.snippet", webdriver ):
+        snippet_btns[ get_tab_for_elem(btn) ].append( btn )
+    # FUDGE! All nationality-specific buttons are created on each OB tab, and the ones not relevant
+    # to each player are just hidden. This is not real good since we have multiple elements with
+    # the same ID :-/ but we work around this by checking if the button is visible. Sigh...
+    snippet_btns2 = {}
+    for tab_id,btns in snippet_btns.items():
+        select_tab( tab_id, webdriver )
+        snippet_btns2[ tab_id ] = [ btn for btn in btns if btn.is_displayed() ]
+    return snippet_btns2
+
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
 def select_droplist_val( sel, val ):
     """Select a droplist option by value."""
     _do_select_droplist( sel, val )
@@ -407,10 +446,10 @@ def dismiss_notifications():
 
 # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
-def click_dialog_button( caption ):
+def click_dialog_button( caption, webdriver=None ):
     """Click a dialog button."""
     btn = next(
-        elem for elem in find_children(".ui-dialog button")
+        elem for elem in find_children( ".ui-dialog button", webdriver )
         if elem.text == caption
     )
     btn.click()
diff --git a/vasl_templates/webapp/vassal.py b/vasl_templates/webapp/vassal.py
index b0d400b..4f702d8 100644
--- a/vasl_templates/webapp/vassal.py
+++ b/vasl_templates/webapp/vassal.py
@@ -126,33 +126,41 @@ def update_vsav(): #pylint: disable=too-many-statements
         },
     } )
 
-def _save_snippets( snippets, fp ):
+def _save_snippets( snippets, fp ): #pylint: disable=too-many-locals
     """Save the snippets in a file.
 
     NOTE: We save the snippets as XML because Java :-/
     """
 
-    def do_save_snippets( webdriver ): #pylint: disable=too-many-locals
-        """Save the snippets."""
-
-        root = ET.Element( "snippets" )
-        for snippet_id,snippet_info in snippets.items():
-
-            # add the next snippet
-            auto_create = "true" if snippet_info["auto_create"] else "false"
-            elem = ET.SubElement( root, "snippet", id=snippet_id, autoCreate=auto_create )
-            elem.text = snippet_info["content"]
-            label_area = snippet_info.get( "label_area" )
-            if label_area:
-                elem.set( "labelArea", label_area )
-
-            # add the raw content
-            elem2 = ET.SubElement( elem, "rawContent" )
-            for node in snippet_info.get( "raw_content", [] ):
-                ET.SubElement( elem2, "phrase" ).text = node
-
-            # include the size of the snippet
-            if webdriver:
+    # NOTE: We used to create a WebDriver here and re-use it for each snippet screenshot,
+    # but when we implemented the shared WebDriver, we changed things to request it for each
+    # snippet. If we did things the old way, the WebDriver wouldn't be able to shutdown
+    # until it had finished *all* the snippet screenshots (since we would have it locked);
+    # the new way, we only have to wait for it to finish the snippet it's on, the WebDriver
+    # will be unlocked, and then the other thread will be able to grab the lock and shut
+    # it down. The downside is that if the user has to disable the shared WebDriver, things
+    # will run ridiculously slowly, since we will be launching a new webdriver for each snippet.
+    # We optimize for the case where things work properly... :-/
+
+    root = ET.Element( "snippets" )
+    for snippet_id,snippet_info in snippets.items():
+
+        # add the next snippet
+        auto_create = "true" if snippet_info["auto_create"] else "false"
+        elem = ET.SubElement( root, "snippet", id=snippet_id, autoCreate=auto_create )
+        elem.text = snippet_info["content"]
+        label_area = snippet_info.get( "label_area" )
+        if label_area:
+            elem.set( "labelArea", label_area )
+
+        # add the raw content
+        elem2 = ET.SubElement( elem, "rawContent" )
+        for node in snippet_info.get( "raw_content", [] ):
+            ET.SubElement( elem2, "phrase" ).text = node
+
+        # include the size of the snippet
+        if not app.config.get( "DISABLE_UPDATE_VSAV_SCREENSHOTS" ):
+            with WebDriver.get_instance() as webdriver:
                 try:
                     start_time = time.time()
                     img = webdriver.get_snippet_screenshot( snippet_id, snippet_info["content"] )
@@ -173,15 +181,8 @@ def _save_snippets( snippets, fp ):
                     logging.error( "Can't get snippet screenshot: %s", ex )
                     logging.error( traceback.format_exc() )
 
-        ET.ElementTree( root ).write( fp )
-        return root
-
-    # save the snippets
-    if app.config.get( "DISABLE_UPDATE_VSAV_SCREENSHOTS" ):
-        return do_save_snippets( None )
-    else:
-        with WebDriver() as webdriver:
-            return do_save_snippets( webdriver )
+    ET.ElementTree( root ).write( fp )
+    return root
 
 def _parse_label_report( fname ):
     """Read the label report generated by the VASSAL shim."""
diff --git a/vasl_templates/webapp/webdriver.py b/vasl_templates/webapp/webdriver.py
index c4541b5..01ba455 100644
--- a/vasl_templates/webapp/webdriver.py
+++ b/vasl_templates/webapp/webdriver.py
@@ -1,26 +1,55 @@
 """ Wrapper for a Selenium webdriver. """
 
 import os
+import threading
 import tempfile
+import atexit
+import logging
 
 from selenium import webdriver
 from PIL import Image, ImageChops
 
-from vasl_templates.webapp import app
+from vasl_templates.webapp import app, cleanup_handlers
 from vasl_templates.webapp.utils import TempFile, SimpleError
 
+_logger = logging.getLogger( "webdriver" )
+
 # ---------------------------------------------------------------------
 
 class WebDriver:
     """Wrapper for a Selenium webdriver."""
 
+    # NOTE: The thread-safety lock controls access to the _shared_instance variable,
+    # not the WebDriver it points to (it has its own lock).
+    _shared_instance_lock = threading.RLock()
+    _shared_instance = None
+
     def __init__( self ):
         self.driver = None
+        self.lock = threading.RLock() # nb: the shared instance must be locked for use
+        self.start_count = 0
+        _logger.debug( "Created WebDriver: %x", id(self) )
+
+    def __del__( self ):
+        try:
+            _logger.debug( "Destroying WebDriver: %x", id(self) )
+        except NameError:
+            pass # nb: workaround a weird crash during shutdown (name 'open' is not defined)
 
     def start( self ):
         """Start the webdriver."""
+        self.lock.acquire()
+        self._do_start()
+
+    def _do_start( self ):
+        """Start the webdriver."""
 
         # initialize
+        self.start_count += 1
+        _logger.info( "Starting WebDriver (%x): count=%d", id(self), self.start_count )
+        if self.start_count > 1:
+            assert self.driver
+            return
         assert not self.driver
 
         # locate the webdriver executable
@@ -36,6 +65,7 @@ class WebDriver:
         # It's pretty icky to have to do this, but since we're in a virtualenv, it's not too bad...
 
         # create the webdriver
+        _logger.debug( "- Launching webdriver process: %s", webdriver_path )
         kwargs = { "executable_path": webdriver_path }
         if "chromedriver" in webdriver_path:
             options = webdriver.ChromeOptions()
@@ -57,14 +87,23 @@ class WebDriver:
             self.driver = webdriver.Firefox( **kwargs )
         else:
             raise SimpleError( "Can't identify webdriver: {}".format( webdriver_path ) )
-
-        return self
+        _logger.debug( "- Started OK." )
 
     def stop( self ):
+        """Stop the webdriver."""
+        self._do_stop()
+        self.lock.release()
+
+    def _do_stop( self ):
         """Stop the webdriver."""
         assert self.driver
-        self.driver.quit()
-        self.driver = None
+        self.start_count -= 1
+        _logger.info( "Stopping WebDriver (%x): count=%d", id(self), self.start_count )
+        if self.start_count == 0:
+            _logger.debug( "- Stopping webdriver process." )
+            self.driver.quit()
+            _logger.debug( "- Stopped OK." )
+            self.driver = None
 
     def get_screenshot( self, html, window_size, large_window_size=None ):
         """Get a preview screenshot of the specified HTML."""
@@ -118,6 +157,52 @@ class WebDriver:
             window_size, window_size2 = window_size2, None
         return self.get_screenshot( snippet, window_size, window_size2 )
 
+    @staticmethod
+    def get_instance():
+        """Return the shared WebDriver instance.
+
+        A Selenium webdriver has a hefty startup time, so we create one on first use, and then re-use it.
+        There are 2 main issues with this approach:
+        - thread-safety: Flask handles requests in multiple threads, so we need to serialize access.
+        - clean-up: it's difficult to know when to clean up the shared WebDriver object. The WebDriver object
+            wraps a chrome/geckodriver process, so we can't just let it leak, since these abandoned processes
+            will just build up. We install atexit and SIGINT handlers, but webdriver processes will still leak
+            if we abend.
+
+        There is a script to stress-test this in the tools directory.
+        """
+
+        # NOTE: We provide a debug switch to disable the shared instance, in case it causes problems
+        # (although things will, of course, run insanely slowly :-/).
+        if app.config.get( "DISABLE_SHARED_WEBDRIVER" ):
+            return WebDriver()
+
+        with WebDriver._shared_instance_lock:
+
+            # check if we've already created the shared WebDriver
+            if WebDriver._shared_instance:
+                # yup - just return it (nb: the caller is responsible for locking it)
+                _logger.info( "Returning shared WebDriver: %x", id(WebDriver._shared_instance) )
+
+                return WebDriver._shared_instance
+
+            # nope - create a new WebDriver instance
+            # NOTE: We start it here to keep it alive even after the caller has finished with it,
+            # and take steps to make sure it gets stopped and cleaned up when the program exits.
+            wdriver = WebDriver()
+            _logger.info( "Created shared WebDriver: %x", id(wdriver) )
+            wdriver._do_start() #pylint: disable=protected-access
+            WebDriver._shared_instance = wdriver
+
+            # make sure the shared WebDriver gets cleaned up
+            def cleanup(): #pylint: disable=missing-docstring
+                _logger.info( "Cleaning up shared WebDriver: %x", id(wdriver) )
+                wdriver._do_stop() #pylint: disable=protected-access
+            atexit.register( cleanup )
+            cleanup_handlers.append( cleanup )
+
+            return wdriver
+
     def __enter__( self ):
         self.start()
         return self