manifest key to enable automatic injection of content scripts after installation/update

[tophf]

Benefits/goals:

• Fixes inconsistency (Chrome doesn't do it, Firefox and Safari do).
• Makes the extension immediately usable.
• Simplifies code: no need for a nontrivial workaround that requires duplicating matches in host_permissions and doesn't support exclude_matches yet (#123).
• Doesn't require reloading the tabs, which may lose their state (games, modern infinite-scroll pages).
• Solves a common pain point for new developers so they don't waste a lot of time in debugger/stackoverflow/etc.

Possible solutions:

1. "run_at": ["document_start", "extension_installed", "extension_enabled"] i.e. an alternative array syntax for run_at
2. "run_at_extension": ["installed", "enabled"] i.e. an additional key
3. "run_at_installed": true and "run_at_enabled": true
4. "run_at_installed_or_enabled": true

Notes:

• In 1 and 2 the browser should disallow invalid/duplicate values.
• The solution should also apply to #600 e.g. "extension_invoked" in 1, "invoked" in 2, "run_at_invoked" in 3 and 4.
• Eventually this should be the default mode, but for now the current default behavior should be explicitly documented:
  • MDN content_scripts should copy the install note from MDN concepts.
  • MDN could add a row in the compat table or add a note about Chromium's behavior.
  • Chrome content_scripts.

[carlosjeurissen]

Very much in favour of having this be aligned cross browsers! Safari also injects content scripts right after installation.

As for the possible solutions. More in favour of 2-4. As 1; using an array for "run_at" would not be backwards compatible. Mixing the two also seems confusing rather than clarifying.

[fregante]

This is also something I wrote some code for (https://github.com/fregante/webext-inject-on-install) but it requires the tabs and scripting permissions.

Related requests:

• injection timing: Request immediate injection of manifest script registered with document_idle #600
• event name: Support onEnabled event when the extension from disabled state to enabled state #353

Here's what they could look like:

  {
    "content_scripts": [{
      "id": "main",
      "matches": ["*://*.example.com/*"],
      "scripts": ["content.js"],
      "run_at": "document_idle",
+     "run_on_active_tab": true,
+     "run_on_extension_enabled": true
    }]
  }

or

  {
    "content_scripts": [{
      "id": "main",
      "matches": ["*://*.example.com/*"],
      "scripts": ["content.js"],
      "run_at": "document_idle",
+     "run_on": ["active_tab", "extension_enabled"]
    }]
  }

or combine it with run_at

  {
    "content_scripts": [{
      "id": "main",
      "matches": ["*://*.example.com/*"],
      "scripts": ["content.js"],
+     "run_at": ["document_idle", "active_tab", "extension_enabled"]
    }]
  }

[carlosjeurissen]

In addition to injection. There is also the removal aspect in case content scripts get registered using registerContentScripts as mentioned in #653.

So we might want to come up with a different naming which also includes the above mentioned behaviour of removing already injected styles (not applying to scripts as injected scripts can not be reversed).