guiMode parameter handling within .JNLP file is partially disregarded by IcedTea-Web 1.8.4 (window maximize unavailable)

Hey folks - running the stock IcedTea-Web 1.8.4 from within AdoptOpenJDK JRE x64 8u292, on Windows 10 1909. Interesting problem here, and would appreciate a second pair of eyes.

**Summary:**
The Windows "maximize" button differs in Oracle Fusion Middleware Forms Services between AdoptOpenJDK JRE (maximize is greyed out) and Oracle Java JRE (maximize is clickable) and is unaffected by guiMode params within the JNLP file. Both JREs appear to not entirely acknowledge guiMode parameters, but equally so. This implies the Windows OS could be affecting or mandating window behavior.

**Steps we took to investigate:**

1. Using a Java Web Start (JNLP)-based website, the .JNLP file is downloaded from Edge Chromium and launches successfully. There are few if any concerns with actual Java Forms functionality - nice!
2. However, users are reporting that content in the Java Forms is being clipped with an internal scrollbar becoming visible within the Oracle Fusion Middleware Forms Services (process name = java.exe). In AdoptOpenJDK, the "Maximize" button in Windows is greyed out. Right-clicking the taskbar or changing OS compatibility modes does not correct or make available the window maximize button.
3. By comparison, in Oracle Java JRE 8u271 the Maximize button is fully available using the same Java Web Start app and JNLP file.
4. In reviewing the differences, we note the frmservlet.JNLP file is explicitly setting [the guiMode parameter](https://docs.oracle.com/middleware/1221/formsandreports/deploy-forms/configure.htm#FSDEP192) to 0.
5. Modfying the guiMode parameter locally within the .JNLP file makes no change within AdoptOpenJDK in terms of fixing the Maximize button, or showing or hiding the Windows title bar. For all possible values, the window bar specifically remains visible and locked in its configuration. The menu bar (File > Edit > Window) does appear to respond correctly to guiMode value changes.
6. Oracle Java 8u271 interestingly appears to disregard guiMode 2 and 3 in the same way as AdoptOpenJDK, and will not remove the Windows title bar under any conditions. Both technically appear to fail the full definition of the guiMode spec, which could be the Windows 10 OS and build overruling how the Java process EXE is trying to render itself.
6. Setting param separateFrame to "true" in AdoptOpenJDK shows a maximizable window within the main window, though the main window is still fixed in size.
7. Changing height and width params within the .JNLP file does allow the window to be sized differently, and is a valid workaround, but does not resolve the overall concern.

Apologies if this is truly coming from the Windows OS as opposed to a quirk of or factor within AdoptOpenJDK and IcedTea-Web, but we do observe a difference in window behavior when performing as close of an apples-to-apples comparison between AdoptOpenJDK JRE and Oracle Java JRE as we are able. I hope this detail helps in reproduction or perhaps a flaw on our side with configuration.

Thank you!