Crash accessing event.srcElement under Firefox when keyboard used while editing

[pastcompute]

mapbox-gl-js version: 0.43.2 but almost certainly any
mapbox-gl-draw version: 1.05, git:master

Steps to Trigger Behavior

1. Use the draw controls
2. Press a key while editing a polygin

Expected Behavior

Either nothing, or an action related to the current mode

Actual Behavior

Stacktrace:

Caught a global error: message=event.srcElement is undefined
customError.ts:5:4
ERROR 
TypeError
​
columnNumber: 1
​
fileName: "http://localhost:8100/build/vendor.js"
​
lineNumber: 186433
​
message: "event.srcElement is undefined"
​
stack: "module.exports/events.keydown@http://localhost:8100/build/vendor.js:186433:1\nF</l</t.prototype.invokeTask@http://localhost:8100/build/polyfills.js:3:15649\nonInvokeTask@http://localhost:8100/build/vendor.js:5114:24\nF</l</t.prototype.invokeTask@http://localhost:8100/build/polyfills.js:3:15562\nF</c</r.prototype.runTask@http://localhost:8100/build/polyfills.js:3:10815\nF</h</e.invokeTask@http://localhost:8100/build/polyfills.js:3:16787\np@http://localhost:8100/build/polyfills.js:2:27646\nv@http://localhost:8100/build/polyfills.js:2:27893\n"
​
__proto__: Object { stack: "", … }

which I tracked down to the following code in src/events.js :

  events.keydown = function(event) {
    if (event.srcElement.classList[0] !== 'mapboxgl-canvas') return; // we only handle events on the map

    if ((event.keyCode === 8 || event.keyCode === 46) && ctx.options.controls.trash) {

but srcElement is not recognised on Firefox: see https://developer.mozilla.org/en-US/docs/Web/API/Event/srcElement

I have a patch, will submit shortly.

[pastcompute]

See also similar issues, e.g. Dash-Industry-Forum/dash.js#590