Netflix interferes with Safari custom tab change shortcut?
Using hammerspoon. After installing hammerspoon (I used brew cask install hammerspoon), just edit/create the file ~/.hammerspoon/init.lua to include:
left = hs.hotkey.new({&quot;cmd&quot;, &quot;alt&quot;}, &quot;left&quot;, function()
  hs.eventtap.keyStroke({&quot;cmd&quot;, &quot;shift&quot;}, &quot;[&quot;)
end)

right = hs.hotkey.new({&quot;cmd&quot;, &quot;alt&quot;}, &quot;right&quot;, function()
  hs.eventtap.keyStroke({&quot;cmd&quot;, &quot;shift&quot;}, &quot;]&quot;)
end)

hs.application.watcher.new(function(appName, eventType, appObject)
  if appName == &quot;Safari&quot; then
    if eventType == hs.application.watcher.activated then
      left:enable()
      right:enable()
    elseif eventType == hs.application.watcher.deactivated then
      left:disable()
      right:disable()
    end
  end
end):start()

Then start Hammerspoon.app.
Then enable Hammerspoon control in System Preferences &gt; Security &amp; Privacy &gt; Privacy &gt; Accessibility. (Maybe you will need to Reload Config from Hammerspoon's menu bar item after this).