A quick preview feels very intuitive & makes the user very comfortable. This feature has been there in Microsoft product & was pointed out by my colleagues when they were migrating to LibreOffice.
Steps to Reproduce:
1. Choose a text for changing the font size.
2. Select different sizes from the drop-down menu.
3. The selected text should change size on the run to preview what size I am about to pick. The change reverts back to previous when the mouse has hovered away from that size.
No preview available
Preview of size change would have helped in deciding what size to select
User Profile Reset: No
This preview is not limited to font size only. Any visual change needs a preview. This will just make life much easier.
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And you should search before reporting.
*** This bug has been marked as a duplicate of bug 37048 ***