Created attachment 119890 [details]
Screenshot of kfontview, scaling factor = 1.7

SUMMARY
The KDE Font Viewer does not display properly on hidpi screens with screen scaling, leaving white margins on the bottom and right side of the font preview area. I use a fractional scaling factor, which is 1.7. The preview part in the font management settings is also influenced. The screenshots of kfontview and the font management settings is shown in the attachments. It seems that the ratio of the total width/height of the font preview area and the width/height of the area actually used is just the scaling factor 1.7.

STEPS TO REPRODUCE
1. Set the screen scaling factor to 1.7 (maybe any number between 1 and 2 is OK) in the display setting;
2. Open the font management settings window or kfontview.

OBSERVED RESULT
White margins in the font preview area.

EXPECTED RESULT
No margins, full area used for font preview.

SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: Kubuntu 19.04
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION
Screen resolution: 2560x1440. Using Intel UHD Graphics 620 graphics card with ubuntu's offical driver.