I use Sumatra 2.3.2 as external viewer for my portable TeXstudio 2.6.2. As command to start Sumatra I tried:
"C:\Program Files (x86)\SumatraPDF\SumatraPDF.exe" -reuse-instance "%.pdf" -forward-search "%.tex" @
dde:///"C:\Program Files (x86)\SumatraPDF\SumatraPDF.exe":SUMATRA/control/[ForwardSearch("?am.pdf","?c:am.tex",@,0,0,1)]
From Sumatra I use:
"C:\<path>\texstudio.exe" "%f" -line %l
When viewing the pdf from TeXstudio, Sumatra starts and shows the correct position. However, a double click in Sumatra starts a second instance of TeXstudio and jumps to the position. Why does Sumatra starts TeXstudio a second time?
To work around this problem one can close the first instance of TeXstudio, which closes Sumatra too. After that it is possible to start Sumatra from TeXstudio again and switch between Sumatra and TeXstudio correctly.