Why forum have a large blank space in the left side ?

Yes, This is happening in large screens.
Small screen users can check with chrome zoom level 50%.

Zoom in / out is not good check case as it's not realistic.. 
What do you understand by "large screen" - on my 24inch monitor its quite normal > same as it is on my MacBook 15  both with max resolutions..

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Perhaps with the new foundation and CI 4 > we can think on some "update" over the forum. 
Easier way would be to keep the forum software with some design updates (this would be quite easier and faster way).
.. Or may-be we can build CI 4 Community app (it can help us to test and find pluses and minuses at the new framework, as app in prod mode with a lot of data). Similar way how CI was initially developed for the EE.

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