Solved
AEM 6.5 core js throws error on dialog open, expects action attribute on .cq-dialog (all.js, line 166)
In line 166 of all.js, there needs to be a check for the action attribute before it tries to get a string match of its value.
This is the existing fragment of code in all.js, line 166:
if (!(0 > c(".coral-Form.cq-dialog").attr("action").indexOf("sticky_menu"))) {
which throws this (blocking) error:
Uncaught TypeError: Cannot read property 'indexOf' of undefined
at HTMLDocument.<anonymous> (all.js:166)
Instead, the line should check that c(".coral-Form.cq-dialog").attr("action") exists before getting its value, like this:
if (c(".coral-Form.cq-dialog").attr("action") && !(0 > c(".coral-Form.cq-dialog").attr("action").indexOf("sticky_menu"))) {
I'm not sure if this error surfaced based on something we've developed, but there should be a check that the element has an attribute before determining its value.