Fix backdrop rootElement not initialized in Modal

[boddunan]

Fixes #33840

The backdrop rootElement is not initialized when the javascript file loaded in the document head as the document.body is not yet initialized and Default variable in backdrop.js is loaded before it.

[GeoSot]

@boddunan Seems a nice solution, but I suppose it would be more valid to solve the root of the problem on backdrop.js

do you want to give it a try?

[boddunan]

Thank you @GeoSot for the feedback. Intializing rootElement with document.body before the document loaded would fail anyway. We have 3 options to fix it as far as I think. Please suggest.

1. Initialize the "Default" variable inside the constructor which is executed after dom load by the plugins
2. Intialize Default.rootElement with document.body before expanding the config values from "Default" object
3. Intialize the Default value of rootElement on DOMContentLoaded, if it is null

Maybe there is some other good alternative, please let me know.

[GeoSot]

Probably I would choose the No2 (Default.rootElement || document.body) or would wait #33327 using the body selector as string

[boddunan]

Thanks. I changed the fix to use method 2. Considering the critical nature of the issue, I would like to fix the issue asap and then apply changes from the other PR when it is merged.

[GeoSot]

Something like this, would be more streamlined with the rest code base

_getConfig(config) {
    
    config = {
      ...Default,
      ...(typeof config === 'object' ? config : {})
}
config .rootElement = config .rootElement || document.body
.....

(rootElement in line 14 better to be null)

Plus, at least a test

[boddunan]

Done. Added tests and moved the intiialization part to _getConfig