It would save me a lot of time if the cells in the tree-view of the explain plan result pane would be individually selectable.
The html tab stays blank btw. (Bug?)

What exactly do you mean by "individually selectable"?

The HTML page seems to work fine. To obtain some more diagnostic information, can you modify the PL/SQL Developer shortcut and add the DebugSQL parameter? For example:

"C:\Program Files\PLSQL Developer 15 Beta\plsqldev.exe" DebugSQL

Reproduce the problem and send me the debug.txt file that is generated in the %APPDATA%\PLSQL Developer 15 directory (e.g. C:\Users\<User>\AppData\Roaming\PLSQL Developer 15).

I can reproduce the blank HTML page in beta 5. I am using dark theme, style=Glow.
Mail sent to support.

Hi Marco, if I "select" a cell in the explain plan tree view and right click I have the option to copy, but that copies the entire explain plan. I'd like to be able to copy an index name, table name or filter predicates.

If I switch the tab from tree to HTML, the html stays empty and stays empty when I re-execute the explain plan. Tabs text and XML display information in both cases.

Okay, thanks for clarifying. We're looking into the blank HTML plan.

From the bug fix list in beta 7:
In the explain plan window, I still get a blank (all-white) HTML tab page.
Should the quoted fix for beta 7 have fixed the explain plan HTML tab? In that case, it still doesn't work...

I am using beta 7, dark theme, style=Dark Outlet Media

This works fine for me in Beta 7 in dark mode, both 32 bit and 64 bit. Can you let me know if you have the "Use internal HTML viewer" option disabled (Preferences > User Interface > Options)?

The option Use internal HTML viewer is disabled. Should I use the Install MS Edge viewer button? I already have Edge and Edge Webview installed.

I can confirm the behavior. If the option "Use internal HTML viewer" is switched off, an empty HTML tab is shown (all white). Even if the MS Edge - Ruintime is installed. At least the installer tells me that WebView2 is already there for all users.
Everything works as expected using the internal viewer.

We'll investigate.