delicate basin
I noticed that if I click the "Multiplayer/Window: NetDbg (Browser)" menu item without my default web browser already running, the Unity Editor freezes until I close the started web browser.
In ves_icall_System_Diagnostics_Process_ShellExecuteEx_internal, when Mono finds the xdg-open program, it sets handler_needswait to TRUE: https://github.com/mono/mono/blob/0339fe117122821856d94dcaa0b08ab966b7ecb2/mono/metadata/w32process-unix.c#L2055
This local variable being set to TRUE later causes that function to synchronously wait for the started process to exit: https://github.com/mono/mono/blob/0339fe117122821856d94dcaa0b08ab966b7ecb2/mono/metadata/w32process-unix.c#L2105
xdg-open waits in turn for the web browser it starts to exit. With most web browsers, if an instance of the browser is already running when clicking the menu item then this isn't much of an issue because the new browser process will quickly communicate with the existing browser instance and immediately exit. However, if there isn't an existing browser instance then the started process will become a "main" instance and will exit when the browser is closed, leaving the Unity Editor frozen until then.
A quick workaround is to have Netcode for Entities directly execute xdg-open on Linux, instead of executing the HTML file and letting Mono see that it failed, search for xdg-open, and if it could find it then execute that with the file as argument and synchronously wait for the process to exit.