Reapplying active font size/family with collapsed selection adds nested spans with same styling

[f1ames]

Are you reporting a feature request or a bug?

Bug

Provide detailed reproduction steps (if any)

1. Go to e.g. /tests/plugins/font/manual/preservestyle (can be performed in any editor instance with font plugin).
2. Add text foo bar.
3. Select whole text.
4. Apply some font size (e.g. 24).
5. Put collapsed selection inside styled text.
6. Apply same font size few times.

Expected result

Font keeps its size. Additional HTML elements are not added.

Actual result

Font keep its size. However, additional nested spans are added and if you start to type you end up with something like <p><span style="font-size:24px">Hel<span style="font-size:24px"><span style="font-size:24px"><span style="font-size:24px"><span style="font-size:24px">foo</span></span></span></span>lo</span> world</p>.

As we updated the logic of dropdown (Default value), previously reapplying the same style just removed it. So it is regression in a sense that it got broken not that it does not work like before.

Other details

• Browser: All (checked on Chrome, FF)
• OS: All
• CKEditor version: 4.8.0
• Installed CKEditor plugins: font

[f1ames]

It might be caused by this changes https://github.com/ckeditor/ckeditor-dev/pull/1056/files#diff-30a76977cbd6d2125b6919b6a952045cR177.