Created attachment 195446 [details]
Video exemplifying the bug manifesting

Reproduction instructions:

1. Ensure your window area is relatively wide (say, wider than the sidebar width + 10 columns).
2. Create a new spreadsheet.
3. Enter some data for a chart, e.g.: 
   
    ab   12
    cd   34

4. Choose the data you entered (4 cells with the example above)
5. Press the Chart toolbar button
6. Click Finish
7. Drag the right edge of the Windows (I'm assuing LTR UI here), to make the window narrower.

Expected results:
Less columns are visible, sidebar visible on right side of window

Actual result: 
Sidebar becomes invisible, and an unused gray area becomes larger, between the actual sheet columns and the sidebar.