2.4+ fails to acquire context in Salesforce Lightning

[kadut]

Inclusion of Chart.js as a static resource for Salesforce Lightning has been broken since 2.4. See for example http://salesforce.stackexchange.com/questions/155591/chartjs-v2-4-0-failed-to-create-chart-cant-acquire-context-from-the-given-item.

Fails with
"Can't acquire context from the given item" - actual line number depends on version

Chrome 56, all extensions disabled to avoid fingerprinting issues.

With a simple component:
<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes">
<ltng:require scripts="/resource/Chart" afterScriptsLoaded="{!c.getChart}"/>

</aura:component>
[note that adding an html id to the canvas does not help]
[
note that there is line a few above here that seems to show up as blank when i look at what i wrote, but should be showing the canvas tag. reproduced below with opening and closing <> left out to make it show up:
canvas aura:id="chart" height="301" width="301"></canvas
]

And controller doing:
var ctx = component.find("chart").getElement().getContext("2d");
var barChart = new Chart(ctx, {type: 'bar', data: data, options: options});

Issue seems to have appeared with #3474 as for some reason the checks fail with Salesforce.

There are 2 lines at issue in function acquireContext(item, config), with reference to 2.4:

if (item instanceof HTMLCanvasElement)
debugger:
false
fix:
if (item instanceof HTMLCanvasElement).nodeName || item.nodeName === "CANVAS")

if (context instanceof CanvasRenderingContext2D)
debugger:
false
fix:
remove this check.

With both these fixes, 2.4 renders my graph above perfectly.
I would have made a PR except that I wasn't sure how you wanted to handle (2) since you have tests using a mock that would fail. I'd use a ducktype check for some method of CanvasRenderingContext2D, but wasn't sure if that was copacetic with your overall scheme.

[etimberg]

@kadut what is Salesforce Lightning actually rendering into? It looks like it replaces all of the DOM with it's own elements that don't actually meet the spec

[kadut]

I am by no means a Salesforce expert, but I believe the wrapping assumption is correct due to their security model, though they claim that the result should be indistinguishable from raw DOM. Not meeting the spec seems a Salesforce deficiency, so arguably you might not want to support it and rather have me but in a request to fix to them, but this package seems pretty popular in the Salesforce world.

[etimberg]

Closing since I don't think this is really a Chart.js error as we're only using standard JS. IMO Salesforce doesn't meet the spec and I think it's up to them to fix.

The message is thrown at https://github.com/chartjs/Chart.js/blob/master/src/core/core.controller.js#L116 and I'm guessing the context is not defined due likely due to https://github.com/chartjs/Chart.js/blob/master/src/platforms/platform.dom.js#L199