diff --git a/circle.yml b/circle.yml
index f93227b..e05aea8 100644
--- a/circle.yml
+++ b/circle.yml
@@ -17,8 +17,8 @@
 #       + https://circleci.com/docs/configuration/
 #   - our `circle.yml` first installs and runs a tunnel to Sauce Labs
 #   - and runs `make server`
-#   - then it calls out to Sauce Labs' REST API to open a browser that reaches
-#     back through the tunnel to access the unit test page on the local server
+#   - then it calls out to Sauce Labs' REST API to open browsers that reach
+#     back through the tunnel to access test pages on the local server
 #       + > Sauce Connect allows you to run a test server within the CircleCI
 #         > build container and expose it it (using a URL like `localhost:8080`)
 #         > to Sauce Labs’ browsers.
@@ -35,167 +35,195 @@
   cache_directories:
     - ~/sauce-connect
   pre:
-    - "test $SAUCE_USERNAME && test $SAUCE_ACCESS_KEY
-       # Sauce Labs credentials required. Sign up here: https://saucelabs.com/opensauce/"
     - ? |-
-        {
-          mkdir -p ~/sauce-connect
-          cd ~/sauce-connect
-          if [ -x sc-*-linux/bin/sc ]; then
-            echo Using cached sc-*-linux/bin/sc
-          else
-            time wget https://saucelabs.com/downloads/sc-latest-linux.tar.gz
-            time tar -xzf sc-latest-linux.tar.gz
-          fi
-          # Sauce Connect randomly fails so try twice
-          time sc-*-linux/bin/sc --user $SAUCE_USERNAME --api-key $SAUCE_ACCESS_KEY --readyfile ~/sauce_is_ready \
-            || time sc-*-linux/bin/sc --user $SAUCE_USERNAME --api-key $SAUCE_ACCESS_KEY --readyfile ~/sauce_is_ready \
-            || echo ERROR > ~/sauce_is_ready
-        } >$CIRCLE_ARTIFACTS/sauce-connect.log 2>&1
+        # SauceConnect: download if not cached, and launch with retry
+        test $SAUCE_USERNAME && test $SAUCE_ACCESS_KEY || {
+          echo 'Sauce Labs credentials required. Sign up here: https://saucelabs.com/opensauce/'
+          exit 1
+        }
+
+        mkdir -p ~/sauce-connect
+        cd ~/sauce-connect
+        if [ -x sc-*-linux/bin/sc ]; then
+          echo Using cached sc-*-linux/bin/sc
+        else
+          time wget https://saucelabs.com/downloads/sc-latest-linux.tar.gz
+          time tar -xzf sc-latest-linux.tar.gz
+        fi
+        # Sauce Connect randomly fails so try twice https://git.io/vPN8v
+        time sc-*-linux/bin/sc --user $SAUCE_USERNAME --api-key $SAUCE_ACCESS_KEY \
+          --readyfile ~/sauce_is_ready
+        test -e ~/sauce_was_ready && exit
+        time sc-*-linux/bin/sc --user $SAUCE_USERNAME --api-key $SAUCE_ACCESS_KEY \
+          --readyfile ~/sauce_is_ready
+        test -e ~/sauce_was_ready && exit
+        echo 'ERROR: Exited twice without creating readyfile' \
+          | tee /dev/stderr > ~/sauce_is_ready
+        exit 1
       :
         background: true
 
 test:
-  override:
-    # Sauce can connect to Safari on ports 3000, 4000, 7000, and 8000. Edge needs port 7000 or 8000.
+  pre:
+    # Safari on Sauce can only connect to port 3000, 4000, 7000, or 8000. Edge needs port 7000 or 8000.
     # https://david263a.wordpress.com/2015/04/18/fixing-safari-cant-connect-to-localhost-issue-when-using-sauce-labs-connect-tunnel/
     # https://support.saucelabs.com/customer/portal/questions/14368823-requests-to-localhost-on-microsoft-edge-are-failing-over-sauce-connect
-    - PORT=8000 make server >$CIRCLE_ARTIFACTS/make_server.log 2>&1:
+    - PORT=8000 make server:
         background: true
 
-    # CircleCI expects test results to be reported in an JUnit/xUnit-style XML
-    # file:
-    #   https://circleci.com/docs/test-metadata/
-    # Our unit tests are in a browser, so they can't write to a file, and Sauce
-    # apparently truncates custom data in their test result reports, so instead
-    # we POST to this trivial Node server on localhost:9000 that writes the
-    # body of any POST request to $CIRCLE_TEST_REPORTS/mocha/xunit.xml
-    - mkdir -p $CIRCLE_TEST_REPORTS/mocha
+    # Wait for tunnel to be ready (`make server` is much faster, no need to wait for it)
+    - while [ ! -e ~/sauce_is_ready ]; do sleep 1; done; touch ~/sauce_was_ready; test -z "$(<~/sauce_is_ready)"
+
+  override:
     - ? |-
-        node << 'EOF' >$CIRCLE_TEST_REPORTS/mocha/xunit.xml \
-                      2>$CIRCLE_ARTIFACTS/mocha-test-report.log
-          require('http').createServer(function(req, res) {
-            res.setHeader('Access-Control-Allow-Origin', '*');
+        # Screenshots: capture in the background while running unit tests
+        mkdir -p $CIRCLE_TEST_REPORTS/mocha
+
+        # CircleCI expects test results to be reported in an JUnit/xUnit-style XML file:
+        #   https://circleci.com/docs/test-metadata/#a-namemochajsamocha-for-nodejs
+        # Our unit tests are in a browser, so they can't write to a file, and Sauce
+        # apparently truncates custom data in their test result reports, so instead we
+        # POST to this trivial Node server on localhost:9000 that writes the body of
+        # any POST request to $CIRCLE_TEST_REPORTS/junit/test-results.xml
+        node -e '
+          require("http").createServer(function(req, res) {
+            res.setHeader("Access-Control-Allow-Origin", "*");
             req.pipe(process.stdout);
-            req.on('end', res.end.bind(res));
+            req.on("end", res.end.bind(res));
           })
           .listen(9000);
-          console.error('listening on http://0.0.0.0:9000/');
-        EOF
-      :
-        background: true
+          console.error("listening on http://0.0.0.0:9000/");
+        ' 2>&1 >$CIRCLE_TEST_REPORTS/junit/test-results.xml | {
+          # ^ note: `2>&1` must precede `>$CIRCLE_TEST_REPORTS/...` because
+          # shell redirect is like assignment; if it came after, then both
+          # stdout and stderr would be written to `xunit.xml` and nothing
+          # would be  piped into here
 
-    # Wait for tunnel to be ready (`make server` and the trivial Node server
-    # are much faster, no need to wait for them)
-    - while [ ! -e ~/sauce_is_ready ]; do sleep 1; done; test "$(<~/sauce_is_ready)" != ERROR
+          head -1 # wait for "listening on ..." to be logged
 
-    # Start taking screenshots in the background while the unit tests are running
-    - ? |-
-        {
+          # https://circleci.com/docs/environment-variables/
+          build_name="CircleCI build #$CIRCLE_BUILD_NUM"
+          if [ $CIRCLE_PR_NUMBER ]; then
+            build_name="$build_name: PR #$CIRCLE_PR_NUMBER"
+            [ "$CIRCLE_BRANCH" ] && build_name="$build_name ($CIRCLE_BRANCH)"
+          else
+            build_name="$build_name: $CIRCLE_BRANCH"
+          fi
+          build_name="$build_name @ ${CIRCLE_SHA1:0:7}"
+          export MQ_CI_BUILD_NAME="$build_name"
+
           time { test -d node_modules/wd || npm install wd; }
           time node script/screenshots.js http://localhost:8000/test/visual.html \
-            && touch ~/screenshots_are_ready || echo ERROR > ~/screenshots_are_ready:
-        } >$CIRCLE_ARTIFACTS/screenshots.log 2>&1
+            && touch ~/screenshots_are_ready || echo EXIT STATUS $? | tee /dev/stderr > ~/screenshots_are_ready:
+        }
       :
         background: true
 
-    # Run in-browser unit tests, based on:
-    #   https://wiki.saucelabs.com/display/DOCS/JavaScript+Unit+Testing+Methods
-    # "build" and "tag" parameters from:
-    #   https://wiki.saucelabs.com/display/DOCS/Test+Configuration+Options#TestConfigurationOptions-TestAnnotation
     - |-
-      curl -i https://saucelabs.com/rest/v1/$SAUCE_USERNAME/js-tests \
-           -X POST \
+      # Unit tests in the browser
+
+      echo '1. Launch tests'
+      echo
+
+      # https://circleci.com/docs/environment-variables/
+      build_name="CircleCI build #$CIRCLE_BUILD_NUM"
+      if [ $CIRCLE_PR_NUMBER ]; then
+        build_name="$build_name: PR #$CIRCLE_PR_NUMBER"
+        [ "$CIRCLE_BRANCH" ] && build_name="$build_name ($CIRCLE_BRANCH)"
+      else
+        build_name="$build_name: $CIRCLE_BRANCH"
+      fi
+      build_name="$build_name @ ${CIRCLE_SHA1:0:7}"
+
+      # "build" and "customData" parameters from:
+      #   https://wiki.saucelabs.com/display/DOCS/Test+Configuration+Options#TestConfigurationOptions-TestAnnotation
+      set -o pipefail
+      curl -i -X POST https://saucelabs.com/rest/v1/$SAUCE_USERNAME/js-tests \
            -u $SAUCE_USERNAME:$SAUCE_ACCESS_KEY \
            -H 'Content-Type: application/json' \
            -d '{
-                 "build": "'$(git rev-parse HEAD)'",
-                 "tags": [
-                   "after-v'$(node -p 'require("./package.json").version')'",
-                   "circle-ci"
-                 ],
+                 "name": "Unit tests, Mocha",
+                 "build": "'"$build_name"'",
+                 "customData": {"build_url": "'"$CIRCLE_BUILD_URL"'"},
                  "framework": "mocha",
                  "url": "http://localhost:8000/test/unit.html?post_xunit_to=http://localhost:9000",
                  "platforms": [["", "Chrome", ""]]
-      }' | tee js-tests
+      }' | tee /dev/stderr | tail -1 > js-tests.json
 
-    # Wait for tests to finish:
-    #
-    #   > Make the request multiple times as the tests run until the response
-    #   > contains `completed: true` to the get the final results.
-    #
-    #   https://wiki.saucelabs.com/display/DOCS/JavaScript+Unit+Testing+Methods
-    - |-
+      echo '2. Wait for tests to finish:'
+      echo
+      #   > Make the request multiple times as the tests run until the response
+      #   > contains `completed: true` to the get the final results.
+      # https://wiki.saucelabs.com/display/DOCS/JavaScript+Unit+Testing+Methods
       while true  # Bash has no do...while >:(
       do
         sleep 5
-        curl -i https://saucelabs.com/rest/v1/$SAUCE_USERNAME/js-tests/status \
-             -X POST \
+        curl -i -X POST https://saucelabs.com/rest/v1/$SAUCE_USERNAME/js-tests/status \
              -u $SAUCE_USERNAME:$SAUCE_ACCESS_KEY \
              -H 'Content-Type: application/json' \
-             -d "$(tail -1 js-tests)" \
-        | tee status
-        tail -1 status > status.json
+             -d @js-tests.json \
+        | tee /dev/stderr | tail -1 > status.json
+
         # deliberately do `... != false` rather than `... == true`
         # because unexpected values should break rather than infinite loop
-        [ "$(node -p 'require("./status.json").completed')" != false ] && break
+        [ "$(jq .completed <status.json)" != false ] && break
       done
 
-    # Wait for screenshots to be ready
-    - while [ ! -e ~/screenshots_are_ready ]; do sleep 1; done; test "$(<~/screenshots_are_ready)" != ERROR:
-        timeout: 300
+      echo '3. Exit with non-zero status code if any unit tests failed'
+      exit "$(jq '.["js tests"][0].result.failures' <status.json)"
 
-    # Stitch together images
-    # TODO: Split this into multiple yaml lines. I (Michael)
-    #       niavely tried to split this into mutiple yaml lines
-    #       but was unsucessful in doing do.
     - |-
-      img_dir=$CIRCLE_ARTIFACTS/imgs/
-      for x in $(ls $img_dir)
-      do
-        convert $img_dir/$x/*.png -append $img_dir/$x.png
+      # Stitch together screenshots and diff against master
+
+      echo '0. Wait for screenshots to be ready'
+      while [ ! -e ~/screenshots_are_ready ]; do sleep 1; done
+      test -z "$(<~/screenshots_are_ready)" || exit 1
+
+      echo '1. Stitch together pieces'
+      for img in $(ls $CIRCLE_ARTIFACTS/imgs/pieces/); do
+        convert $(ls -1 $CIRCLE_ARTIFACTS/imgs/pieces/$img/*.png | sort -n) -append $CIRCLE_ARTIFACTS/imgs/$img.png
       done
 
-      # Remove all directories in $CIRCLE_ARTIFACTS/img
-      # Currently the pieces aren't kept around. If it's
-      # desirable to keep them around, we should use
-      #    cp -r $dir $CIRCLE_ARTIFACTS/img_pieces
-      # The reason the image pieces aren't currently kept
-      # around is that it was leading to a problem. Specifically,
-      # when we get the previous images, we niavely grab any *.png,
-      # including the pieces images. This compounded so that each
-      # iteration of a test run would have all of the images from
-      # the previous test run plus whichever new images were generated.
-      rm -R -- $img_dir/*/
+      echo '2. Download the latest screenshots from master'
+      echo
 
-      # Install utility we need
-      npm install -g json
+      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=ci.cleanup)"
+      #when done testing, restore: artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=master)"
+      echo
+      echo '/latest/artifacts?branch=master:'
+      echo
+      echo "$artifacts_json"
+      echo
 
-      # Download the latest mathquill artifacts.
-      curl $(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
-              | json -a url pretty_path -d '\n\t' \
-              | grep '\.png$' \
-              | grep -v 'PREV' \
-              | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $img_dir/PREV_:")
+      mkdir $CIRCLE_ARTIFACTS/imgs/baseline/
+      baseline_imgs="$(echo "$artifacts_json" \
+                    | jq -r '.[] | .url + " -o " + .pretty_path' \
+                    | grep '\.png$' \
+                    | grep -v '_DIFF\.png$' \
+                    | grep -vF '/pieces/' \
+                    | grep -vF '/baseline/' \
+                    | sed "s:\$CIRCLE_ARTIFACTS/imgs/:$CIRCLE_ARTIFACTS/imgs/baseline/:")"
+      echo 'Baseline image URLs and files:'
+      echo
+      echo "$baseline_imgs"
+      echo
 
-      # Generate image diffs.
-      cd $img_dir
-      for file in $(ls PINNED*); do
-        prev=PREV_$file
-        metric_diff=$(compare -metric AE -compose src $prev $file raw_diff.png)
-        composite -alpha on raw_diff.png $prev DIFF_$file
+      test -z "$baseline_imgs" && { echo 'No baseline images to download'; exit; }
+      curl $baseline_imgs
+      echo
+
+      echo '3.  Generate image diffs'
+      echo
+      cd $CIRCLE_ARTIFACTS/imgs/
+      for file in $(ls *.png); do
+        # if evergreen browser, browser version of previous screenshot may not match,
+        # so replace previous browser version with glob
+        baseline="$(echo baseline/$(echo $file | sed 's/[^_]*_(evergreen)/*/; s/OS_X_.*/OS_X_*.png/' | tee /dev/stderr) | tee /dev/stderr)"
+        echo "Number of different pixels from baseline in $file:"
+        compare -metric AE $baseline $file ${file/%.png/_DIFF.png}
+        echo
       done
+      true  # ignore errors like "image widths or heights differ"
 
-      for file in $(ls EVERGREEN*); do
-        prev=$(ls PREV_$file*)
-        metric_diff=$(compare -metric AE -compose src $prev $file raw_diff.png)
-        composite -alpha on raw_diff.png $prev DIFF_$file
-      done
-
-      rm raw_diff.png
-
-    # finally, complain to Circle CI if there were nonzero test failures
-    - |-
-      [ "$(node -p 'require("./status.json")["js tests"][0].result.failures')" == 0 ]
   post:
     - killall --wait sc; true  # wait for Sauce Connect to close the tunnel; ignore errors since it's just cleanup
diff --git a/script/screenshots.js b/script/screenshots.js
index a28f035..63d9374 100644
--- a/script/screenshots.js
+++ b/script/screenshots.js
@@ -14,186 +14,193 @@
 
 var wd = require('wd');
 var fs = require('fs');
-var username = process.env['SAUCE_USERNAME'];
-var accessKey = process.env['SAUCE_ACCESS_KEY'];
-var baseDir = process.env['CIRCLE_ARTIFACTS'] || '/tmp';
 var url = process.argv[2];
-var allImgsDir = baseDir+'/imgs';
-fs.mkdirSync(allImgsDir);
+var username = process.env.SAUCE_USERNAME;
+var accessKey = process.env.SAUCE_ACCESS_KEY;
+var build_name = process.env.MQ_CI_BUILD_NAME;
+var baseDir = process.env.CIRCLE_ARTIFACTS;
+if (!baseDir) {
+  console.error('No $CIRCLE_ARTIFACTS found, for testing do something like `CIRCLE_ARTIFACTS=/tmp script/screenshots.js`');
+  process.exit(1);
+}
+fs.mkdirSync(baseDir+'/imgs');
+fs.mkdirSync(baseDir+'/imgs/pieces');
+fs.mkdirSync(baseDir+'/browser_logs');
 
-var browserVersions = [
+var browsers = [
   {
-    'version': {
-      // Expecting IE 8
-      'browserName': 'Internet Explorer',
-      'platform': 'Windows XP'
+    config: {
+      browserName: 'Internet Explorer',
+      platform: 'Windows XP'
     },
-    'pinned': 'PINNED'
+    pinned: true // assume pinned to IE 8
   },
   {
-    'version': {
-      // Expecting IE 11
-      'browserName': 'Internet Explorer',
-      'platform': 'Windows 7'
+    config: {
+      browserName: 'Internet Explorer',
+      platform: 'Windows 7'
     },
-    'pinned': 'PINNED'
+    pinned: true // assume pinned to IE 11
   },
   {
-    'version': {
-      'browserName': 'MicrosoftEdge',
-      'platform': 'Windows 10'
-    },
-    'pinned': 'EVERGREEN'
+    config: {
+      browserName: 'MicrosoftEdge',
+      platform: 'Windows 10'
+    }
   },
   {
-    'version': {
-      'browserName': 'Firefox',
-      'platform': 'OS X 10.11'
-    },
-    'pinned': 'EVERGREEN'
+    config: {
+      browserName: 'Firefox',
+      platform: 'OS X 10.11'
+    }
   },
   {
-    'version': {
-      'browserName': 'Safari',
-      'platform': 'OS X 10.11'
-    },
-    'pinned': 'EVERGREEN'
+    config: {
+      browserName: 'Safari',
+      platform: 'OS X 10.11'
+    }
   },
   {
-    'version': {
-      'browserName': 'Chrome',
-      'platform': 'OS X 10.11'
-    },
-    'pinned': 'EVERGREEN'
+    config: {
+      browserName: 'Chrome',
+      platform: 'OS X 10.11'
+    }
   },
   {
-    'version': {
-      'browserName': 'Firefox',
-      'platform': 'Linux'
-    },
-    'pinned': 'EVERGREEN'
-  },
+    config: {
+      browserName: 'Firefox',
+      platform: 'Linux'
+    }
+  }
 ];
 
 
-browserVersions.forEach(function(obj) {
-  var cfg = obj.version;
-  var browserDriver = wd.remote('ondemand.saucelabs.com', 80, username, accessKey);
-  // The following is in the style of
-  // https://github.com/admc/wd/blob/62f2b0060d36a402de5634477b26a5ed4c051967/examples/async/chrome.js#L25-L40
-  browserDriver.init(cfg, function(err, _, capabilities) {
-    if (err) console.log(err);
-    console.log(cfg.browserName,cfg.platform,'init')
+browsers.forEach(function(browser) {
+  browser.config.build = build_name;
+  browser.config.name = 'Visual tests, ' + browser.config.browserName + ' on ' + browser.config.platform;
+  browser.config.customData = {build_url: process.env.CIRCLE_BUILD_URL};
+  var browserDriver = wd.promiseChainRemote('ondemand.saucelabs.com', 80, username, accessKey);
+  return browserDriver.init(browser.config)
+  .then(function(args) {
+    var cfg = browser.config, capabilities = args[1];
+    var version = capabilities.version || capabilities.browserVersion;
+    var sessionName = [cfg.browserName, version, cfg.platform].join(' ');
+    if (capabilities.platformVersion) sessionName += ' ' + capabilities.platformVersion;
+    console.log(sessionName, 'init', args);
 
-    var browser = cfg.browserName.replace(/\s/g, '_');
-    var platform = cfg.platform.replace(/\s/g, '_');
-    var piecesDir = allImgsDir+'/'+obj.pinned+'_'+platform+'_'+browser;
-    fs.mkdirSync(piecesDir);
+    var evergreen = browser.pinned ? '' : '_(evergreen)';
+    var fileName = [cfg.browserName, version + evergreen, cfg.platform].join('_');
+    if (capabilities.platformVersion) fileName += ' ' + capabilities.platformVersion;
+    fileName = fileName.replace(/ /g, '_');
 
-    browserDriver.get(url, function(err) {
-      if (err) console.log(err);
-      console.log(cfg.browserName,cfg.platform,'get')
-      browserDriver.safeExecute('document.documentElement.scrollHeight', function(err,scrollHeight) {
-        if (err) console.log(err);
-        console.log(cfg.browserName,cfg.platform,'get scrollHeight')
-        browserDriver.safeExecute('document.documentElement.clientHeight', function(err,viewportHeight) {
-          if (err) console.log(err);
-          console.log(cfg.browserName,cfg.platform,'get clientHeight')
+    return browserDriver.get(url)
+    .then(willLog(sessionName, 'get'))
+    .safeExecute('document.body.focus()') // blur anything that's auto-focused
+    .then(willLog(sessionName, 'document.body.focus()'))
+    .safeExecute('document.documentElement.style.overflow = "hidden"') // hide scrollbars
+    .then(willLog(sessionName, 'hide scrollbars'))
+    .then(function() {
+      // Microsoft Edge starts out with illegally big window: https://git.io/vD63O
+      if (cfg.browserName === 'MicrosoftEdge') {
+        return browserDriver.getWindowSize()
+        .then(function(size) {
+          return browserDriver.setWindowSize(size.width, size.height)
+        })
+        .then(willLog(sessionName, 'reset window size (Edge-only workaround)'))
+      }
+    })
+    .then(function() {
+      return [browserDriver.safeExecute('document.documentElement.scrollHeight'),
+              browserDriver.safeExecute('document.documentElement.clientHeight')];
+    })
+    .spread(function(scrollHeight, viewportHeight) {
+      console.log(sessionName, 'get scrollHeight, clientHeight', scrollHeight, viewportHeight);
 
-          // Firefox and Internet Explorer will take a screenshot of the entire webpage,
-          if (cfg.browserName != 'Safari' && cfg.browserName != 'Chrome' && cfg.browserName != 'MicrosoftEdge') {
-            // saves file in the file `piecesDir/browser_version_platform/*.png`
-            var filename = piecesDir+'/'+browser+'_'+platform+'.png';
-            browserDriver.saveScreenshot(filename, function(err) {
-              if (err) console.log(err);
-              console.log(cfg.browserName,cfg.platform,'saveScreenshot');
+      // the easy case: Firefox and IE return a screenshot of the entire webpage
+      if (cfg.browserName === 'Firefox' || cfg.browserName === 'Internet Explorer') {
+        return browserDriver.saveScreenshot(baseDir + '/imgs/' + fileName + '.png')
+        .then(willLog(sessionName, 'saveScreenshot'))
+      // the hard case: for Chrome, Safari, and Edge, scroll through the page and
+      // take screenshots of each piece; circle.yml will stitch them together
+      } else {
+        var piecesDir = baseDir + '/imgs/pieces/' + fileName + '/';
+        fs.mkdirSync(piecesDir);
 
-              browserDriver.log('browser', function(err,logs) {
-                if (err) console.log(err);
-                console.log(cfg.browserName,cfg.platform,'log');
+        var scrollTop = 0;
+        var index = 1;
 
-                var logfile = baseDir+'/'+browser+'_'+platform+'.log'
-                logs = logs || [];
-                fs.writeFile(logfile,logs.join('\n'), function(err) {
-                  if (err) console.log(err);
+        return (function loop() {
+          return browserDriver.safeEval('window.scrollTo(0,'+scrollTop+');')
+          .then(willLog(sessionName, 'scrollTo()'))
+          .saveScreenshot(piecesDir + index + '.png')
+          .then(function() {
+            console.log(sessionName, 'saveScreenshot');
 
-                  browserDriver.quit();
-                });
-              });
+            scrollTop += viewportHeight;
+            index += 1;
 
-            });
-          } else {
-            var scrollTop = 0;
-
-            // loop generates the images. Firefox and Internet Explorer will take
-            // a screenshot of the entire webpage, but Opera, Safari, and Chrome
-            // do not. For those browsers we scroll through the page and take
-            // incremental screenshots.
-            (function loop() {
-              var index = (scrollTop/viewportHeight) + 1;
-
-              // Use `window.scrollTo` because thats what jQuery does
-              // https://github.com/jquery/jquery/blob/1.12.3/src/offset.js#L186
-              // `window.scrollTo` was used instead of jQuery because jQuery was
+            // if the viewport hasn't passed the bottom edge of the page yet,
+            // scroll down and take another screenshot
+            if (scrollTop + viewportHeight <= scrollHeight) {
+              // Use `window.scrollTo` because thats what jQuery does:
+              //   https://github.com/jquery/jquery/blob/1.12.3/src/offset.js#L186
+              // Use `window.scrollTo` instead of jQuery because jQuery was
               // causing a stackoverflow in Safari.
-              browserDriver.safeEval('window.scrollTo(0,'+scrollTop+');', function(err) {
-                if (err) console.log(JSON.stringify(err));
-                console.log(cfg.browserName,cfg.platform,'safeEval 1');
+              return loop();
+            } else { // we are past the bottom edge of the page, reduce window size to
+              // fit only the part of the page that hasn't been screenshotted.
 
-                // saves file in the file `piecesDir/browser_version_platform/#.png`
-                var filename = piecesDir+'/'+index+'.png';
-                browserDriver.saveScreenshot(filename, function(err) {
-                  if (err) console.log(err);
-                  console.log(cfg.browserName,cfg.platform,'saveScreenshot');
+              // If there is no remaining part of the page, we're done, short-circuit
+              if (scrollTop === scrollHeight) return browserDriver;
 
-                  scrollTop += viewportHeight;
-                  if (scrollTop + viewportHeight > scrollHeight) {
-                    browserDriver.getWindowSize(function(err,size) {
-                      if (err) console.log(err);
-                      console.log(cfg.browserName,cfg.platform,'getWindowSize');
-                      // account for the viewport offset
-                      var extra = size.height - viewportHeight;
-                      browserDriver.setWindowSize(size.width, (scrollHeight-scrollTop)+extra, function(err) {
-                        if (err) console.log(err);
-                        console.log(cfg.browserName,cfg.platform,'setWindowSize');
-
-                        browserDriver.safeEval('window.scrollTo(0,'+scrollHeight+');', function(err) {
-                          if (err) console.log(JSON.stringify(err));
-                          console.log(cfg.browserName,cfg.platform,'safeEval 2');
-
-                          index++;
-                          var filename = piecesDir+'/'+index+'.png';
-                          browserDriver.saveScreenshot(filename, function(err) {
-                            if (err) console.log(err);
-                            console.log(cfg.browserName,cfg.platform,'saveScreenshot Final');
-
-                            browserDriver.log('browser', function(err,logs) {
-                              if (err) console.log(err);
-                              console.log(cfg.browserName,cfg.platform,'log');
-
-                              var logfile = baseDir+'/'+browser+'_'+platform+'.log'
-                              logs = logs || [];
-                              fs.writeFile(logfile,logs.join('\n'), function(err) {
-                                if (err) console.log(err);
-                                console.log(cfg.browserName,cfg.platform,'writeFile');
-
-                                browserDriver.quit();
-                              });
-                            });
-                          });
-                        });
-
-                      });
-                    });
-                  } else {
-                    loop();
-                  }
-                });
+              return browserDriver.getWindowSize()
+              .then(function(windowSize) {
+                console.log(sessionName, 'getWindowSize');
+                // window size is a little bigger than the viewport because of address
+                // bar and scrollbars and stuff
+                var windowPadding = windowSize.height - viewportHeight;
+                var newWindowHeight = scrollHeight - scrollTop + windowPadding;
+                return browserDriver.setWindowSize(windowSize.width, newWindowHeight)
+                .then(willLog(sessionName, 'setWindowSize'))
+                .safeEval('window.scrollTo(0,'+scrollHeight+');')
+                .then(willLog(sessionName, 'scrollTo() Final'))
+                .saveScreenshot(piecesDir + index + '.png')
+                .then(willLog(sessionName, 'saveScreenshot Final'));
               });
-            })();
-          }
-        });
+            }
+          });
+        }());
+      }
+    })
+    .then(function() {
+      return browserDriver.log('browser')
+      .then(function(logs) {
+        var logfile = baseDir + '/browser_logs/' + sessionName.replace(/ /g, '_') + '.log';
+        return new Promise(function(resolve, reject) {
+          fs.writeFile(logfile, JSON.stringify(logs, null, 2), function(err) {
+            err ? reject(err) : resolve();
+          });
+        })
+        .then(willLog(sessionName, 'writeFile'));
+      }, function(err) {
+        // the Edge, IE, and Firefox-on-macOS drivers don't support logs, but the others do
+        console.log(sessionName, 'Error fetching logs:', JSON.stringify(err, null, 2));
       });
     });
-  });
+  })
+  .sauceJobStatus(true)
+  .fail(function(err) {
+    console.log('ERROR:', browser.config.browserName, browser.config.platform);
+    console.log(JSON.stringify(err, null, 2));
+    return browserDriver.sauceJobStatus(false);
+  })
+  .quit();
+
+  function willLog() {
+    var msg = [].join.call(arguments, ' ');
+    return function(value) {
+      console.log(msg);
+      return value;
+    };
+  }
 });