diff --git a/features/steps/output_steps.rb b/features/steps/output_steps.rb
index 47ceeb8..ce9a12a 100644
--- a/features/steps/output_steps.rb
+++ b/features/steps/output_steps.rb
@@ -27,6 +27,17 @@ Then /^the output must not match \/([^\/]+)\/([a-z]*)$/ do |pattern, options|
   expect(all_output).not_to match Regexp.new(pattern, options)
 end
 
+Then /^the output must match \/([^\/]+)\/([a-z]*) at least (\d+) times$/ do
+    |pattern, options, times|
+  uhwm_wait_output Regexp.new(pattern, options), times.to_i
+end
+
+Then /^the output must match \/([^\/]+)\/([a-z]*) exactly (\d+) times$/ do
+    |pattern, options, times|
+  scans = uhwm_wait_output Regexp.new(pattern, options)
+  expect(scans.size).to eq times.to_i
+end
+
 Then /^the output must contain:$/ do |content|
   uhwm_wait_output content.to_s
 end
diff --git a/features/steps/x_steps.rb b/features/steps/x_steps.rb
index fcdc8a8..fff2472 100644
--- a/features/steps/x_steps.rb
+++ b/features/steps/x_steps.rb
@@ -9,6 +9,10 @@ When /^I press the ([^ ]+) keys?$/ do |keys|
   x_key keys
 end
 
+When /^I press the ([^ ]+) keys? (\d+) times$/ do |keys, times|
+  times.to_i.times { x_key keys }
+end
+
 When /^a window requests to be mapped$/ do
   x_client.map.sync
 end
diff --git a/features/workers/block.feature b/features/workers/block.feature
new file mode 100644
index 0000000..b8cb86f
--- /dev/null
+++ b/features/workers/block.feature
@@ -0,0 +1,15 @@
+Feature: blocking worker
+
+  Scenario: processes initial events
+    Given uhwm is running with options -d -w block
+    Then the output must match /xevent/i at least 2 times
+
+  Scenario: processes generated events
+    Given a run control file with:
+      """
+      key(:f) { puts 'testing_worker_read' }
+      """
+    And uhwm is running with options -d -w block
+    When I press the alt+f key 3 times
+    And I quit uhwm
+    Then the output must match /(testing_worker_read)/ exactly 3 times
diff --git a/lib/uh/wm/testing/acceptance_helpers.rb b/lib/uh/wm/testing/acceptance_helpers.rb
index 7558ca0..4d56e83 100644
--- a/lib/uh/wm/testing/acceptance_helpers.rb
+++ b/lib/uh/wm/testing/acceptance_helpers.rb
@@ -29,19 +29,21 @@ module Uh
           end
         end
 
-        def uhwm_wait_output message
+        def uhwm_wait_output message, times = 1, value = nil
           output = -> { @process.stdout + @process.stderr }
           timeout_until do
             case message
-              when Regexp then output.call =~ message
-              when String then output.call.include? message
+            #when Regexp then output.call.scan(message).size >= times
+            when Regexp then (value = output.call.scan(message)).size >= times
+            when String then output.call.include? message
             end
           end
+          value
         rescue TimeoutError => e
-          fail [
-            "expected `#{message}' not seen after #{e.timeout} seconds in:",
-            "  ```\n#{output.call.lines.map { |e| "  #{e}" }.join}  ```"
-          ].join "\n"
+          fail <<-eoh
+expected `#{message}' (#{times}) not seen after #{e.timeout} seconds in:
+  ```\n#{output.call.lines.map { |e| "  #{e}" }.join}  ```
+          eoh
         end
 
         def uhwm_wait_ready