I noticed I cannot export product categories while exporting account.invoice records. I dig into the code a bit and found this:
tabindex="-1"> <!-- tabindex make the div focusable -->
<span t-if="field.children && (field.id).split('/').length != 3"
class="o_expand_parent fa fa-plus"/>
<span class="o_tree_column"><t t-esc="field.string"/></span>
I removed this condition that check how many text block there is after splitting with "/" character, which is how many levels:
&& (field.id).split('/').length != 3
But still, it doesn't show any + signs after 3 levels :(
I suspect in the code something that would set field.children to false when getting to level 3, I don't know.
My next question will be how to make a module that change this. I want to allow drilling to 5 levels.