Created attachment 161748 [details]
Screenshot of window with only option to minimize, maximize

SUMMARY
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I believe this specific window should, by default, have the option to be closeable in case I click on a link and it is asking to open in an Application. In this case, I was always planning on using the browser. (On Steam Deck , latest stable version) 
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STEPS TO REPRODUCE
1. From Firefox, Click on a link that requests opening in an installed app, like Discord. Here is my example : https://discord.com/channels/506164002003484673/1153363570659180696
2. Window opens asking for application to open with, or to view list of apps in Discover


OBSERVED RESULT
Window opens asking for application or offering to open Discover to open the link in an application

EXPECTED RESULT
Window opens asking for application or offering to open Discover to handle the link, but I am able to close that window. 

SOFTWARE/OS VERSIONS
Operating System: SteamOS 3.4.10
KDE Plasma Version: 5.26.1
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.6
Kernel Version: 5.13.0-valve37-1-neptune (64-bit)
Graphics Platform: X11
Processors: 8 × AMD Custom APU 0405
Memory: 14.5 GiB of RAM
Graphics Processor: AMD Custom GPU 0405
Manufacturer: Valve
Product Name: Jupiter
System Version: 1

ADDITIONAL INFORMATION
Workaround: 
1. Right click window > More Actions > Configure special Application settings > Add Property > Closeable = Yes