Some labels are not fully represented in table viewer (Tab General).
Se picture in Attachment.

Have you perhaps changed the "Main font" (Preferences > User Interface > Fonts)? If so, can you reset it to "Segoe UI" and try again?

I hadn't made any change. However, the default font does not seem to have been used.
The font was probably changed in an earlier version. I can see the same behavior in v14 as well. I just hadn't noticed. Like I said, nothing important.
After switching to "Segoe UI" it looks much better. However, there is one very small thing (see picture).

I don't see this. Can you go to Help > Support Info and send me the text from the "Info" tab page?

Here is the content of the info page:

PL/SQL Developer
Version 15.0.0.2046 Beta (64 bit)
Theme: Midnight Express / Office2019Black
Windows 10 Build 19043
de-DE(1031)/Deutsch (Deutschland)

Physical memory : 4.193.780 kB (797.712 available)
Paging file : 7.162.060 kB (1.383.652 available)
Virtual memory : 137.438.953.344 kB (137.433.916.744 available)

Parameters
E:\prg\db\tools\plsd15\plsqldev.exe

Preferences
Session mode: Dual
OCI Library: <none>
Use OCI7: False
Allow Multiple Connections: False

Preference Files
E:\prg\db\tools\plsd15\Preferences\Default\Default.ini
C:\Users\frankg\AppData\Roaming\PLSQL Developer 15\Preferences\frankg\default.dark.ini

Debug file
E:\prg\db\tools\plsd15\PlSqlDev.elf

Plug-Ins
*Active Query Builder (E:\prg\db\tools\plsd15\PlugIns\ActiveQueryBuilder.dll)
*PL/SQL Documentation (plsqldoc) (E:\prg\db\tools\plsd15\PlugIns\plsqldoc.dll)
(* is Active)

Aliases
adbfg1_high
adbfg1_low
adbfg1_medium
adbfg1_tp
adbfg1_tpurgent
...

Homes
OraClient19Home1 (E:\oracle\product\19.0.0\dbhome_1)

DLLs
E:\oracle\product\19.0.0\dbhome_1\bin\oci.dll

TNS File
E:\Oracle\product\19.0.0\dbhome_1\network\admin\tnsnames.ora

Using
Home: OraClient19Home1
DLL: E:\oracle\product\19.0.0\dbhome_1\bin\oci.dll
OCI: version 12.1 (19.3.0.0.0)
Oracle Database 12c Standard Edition Release 12.2.0.1.0

Character Sets
Character size: 4 byte(s)
CharSetID: 873
NCharSetID: 2000
Unicode Support: True
NLS_LANG: GERMAN_GERMANY.WE8MSWIN1252
NLS_NCHAR_CHARACTERSET: AL16UTF16
NLS_CHARACTERSET: AL32UTF8

Process
Working Set = 169.029.632
Memory = 145.221.674
GDI Objects = 1391
User Objects = 7040
Handles = 1100

Monitor
PixelsPerInch = 168

Id = 0
PPI = 168
Primary = True
Handle = 65537
Left = 0
Top = 0
Width = 2871
Height = 2040

MainFormOnTaskbar = True

I see you are using 175% display scaling. We'll test with that.

Wow, I couldn't have read the scaling factor from the support data.
But this is really a very, very small blemish. And no longer annoying after changing the font to Segoe UI".