When pressing the command button, I see this error (due to Bug 1128988): Error: environment.chromeWindow is not available in runAt:server commands Stack trace: GcliActor<._getRequisition/this._requisitionPromise</environment.chromeWindow@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/gcli.js:256:17 exports.items<.exec@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/gcli/commands/rulers.js:59:9 Requisition.prototype.exec/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/gcli/cli.js:2073:16 createAsyncFunction/asyncFunction@resource://gre/modules/Task.jsm:239:18 Task_spawn@resource://gre/modules/Task.jsm:164:12 exports.exec@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/gcli/util/host.js:72:10 Requisition.prototype.exec@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/gcli/cli.js:2072:14 Requisition.prototype.updateExec/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/gcli/cli.js:2112:12 Handler.prototype.process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:867:23 this.PromiseWalker.walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:746:7 this.PromiseWalker.scheduleWalkerLoop/<@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:688:37 Promise*this.PromiseWalker.scheduleWalkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:688:5 this.PromiseWalker.schedulePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:709:7 Promise.prototype.then@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:417:5 GcliActor<.execute<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/actors/gcli.js:116:12 actorProto/</handler@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/protocol.js:1006:19 DSC_onPacket@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/main.js:1456:15 LocalDebuggerTransport.prototype.send/<@resource://gre/modules/devtools/dbg-client.jsm -> resource://gre/modules/devtools/transport/transport.js:561:11 makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/DevToolsUtils.js:82:14 makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/DevToolsUtils.js:82:14

I suspect that the solution for bug 1164537 would fit here.

Does that make sense Matteo?

More sprinkles of runAt. Try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=5f86d9a82db2

This command should run on the server, not the client. The ruler is a kind of highlighter (see toolkit/devtools/server/actors/highlighter.js), and as such, it needs to inject its content into the native anonymous content in the page, so it really needs access to the page, especially to detect scroll and refresh when it happens (with the patch applied, the ruler doesn't manage to scroll correctly). So this command should instead work like the highlight command, which runs on the server. I'm in the process of cleaning the highlight command a little bit in bug 1147826 to avoid having to depend on the tabActor.

I might have a fix for this in bug 1147826. I need to work on it some more, but if it turns out to be working, I'll mark this one as duplicate.

That sounds like a reasonable approach! I'll leave this one alone then.