Chrome extension Content Script not loaded until page is refreshed

Solution 1:

The problem was that Trello uses HTML5's pushState for page transitions, so the content script wasn't always being run after a board was opened.

Solution

Changes to manifest:

{
  "manifest_version": 2,

  "name": "Temp Ext",
  "version": "1.1",

  "content_scripts": [{
    "matches": ["*://trello.com/*"],
    "js":["contentscript.js"]
  }],

  "background": {
    "scripts": ["background.js"]
  },

  "permissions": [
    "*://trello.com/*", "tabs", "webNavigation"
  ]
}

Add background script:

chrome.webNavigation.onHistoryStateUpdated.addListener(function(details) {
    chrome.tabs.executeScript(null,{file:"contentscript.js"});
});