I had the same issue as the original poster: double click on Windows 10 would not open a Layout 2018 file if Layout was already open.
My fix was to run Layout.exe as administrator.
I’m unsure why this worked. The original installation was undertaken as administrator.
I only have 2018 installed, however the PC did have 2017 at one point in time. Layout 2017 still appears as an option if I right click open as, suggesting is something amiss with remnants of 2017 or file association.
I have had a similar issue. I can open any file initially, but once Layout is running, the only way to open a file is to go through the open tab in Layout. I have only had Sketchup 2018. Running Layout.exe as admin didn’t work as a fix.
Running SketchUp or Layout as administrator is not recommended.
Because this elevates their exe file’s security-level, and then things like drag-and-drop will no longer work, because Windows Explorer always runs at a base security-level, and a lower process cannot affect a higher process.
However, if you have not installed SketchUp properly then running it as admin CAN resolve some permission problems that the poor installation causes…
BUT you are better off correctly installing SketchUp.
It’s possible to ‘Repair’ the installation as well.
Here’s the fix…
Log in to your usual Windows user account.
You do NOT need admin powers, but if you have them it doesn’t matter.
BUT do not log in as the system admin - just use your normal user account.
Close SketchUp if it’s open.
Find the SketchUp installer exe file - it’s probably in your Downloads folder.
Select its icon, right-click > context-menu > “Run as administrator”
When prompted choose ‘Repair’…
When it completes check the Properties of the SketchUp.exe and Layout.exe files in the ‘Program Files’ subfolder[s]… and ensure they are NOT set to ‘always run as an administrator’ under the ‘Compatibility’ tab.
Curiously, knowing I had installed correctly , I went to check the compatibility tab. Run as admin was not checked so…I checked it. Then I went to a layout file and DC’ed and selected another. Both files opened in the same instance of layout. Next I went to SU, Sent a file to a closed layout then sent a second scene. Both opened in the same window under different tabs. Both in '18 and '19