Investigation into bug 35862 - allow "increase/decrease font" for text selection with multiple font sizes

Hi devs,

I began investigating this bug: 
https://bugs.freedesktop.org/show_bug.cgi?id=35862

The bug is that the "Increase Font"/"Decrease Font" button should be 
useable when one
selects text of multiple sizes in Writer.

I tried to find where the button was being used and found that the 
button for, for example,
increasing the font size has an ID referenced as FN_GROW_FONT_SIZE. I 
put print statements
for any references I found and was able to find the code that is 
executed when the button
is clicked, but I don't know what code activates/deactivates the actual 
buttons. I suspect
that since this functionality is available in Impress but not Writer 
(which is what the bug
is about), there could be some clue if we consider where the code 
between Writer/Impress differs.

Currently I suspect the file svx/sdi/svx.sdi might have information, in 
the definition of

SfxVoidItem Grow SID_GROW_FONT_SIZE

but I don't know how this hooks in with the application.

As I am new to LibreOffice, I'm wondering if anyone has any ideas where 
I can look next?

Thanks!
Jordan