The plus and remove emblem icons in Marlin are pretty insensitive. Combined with single-click open/launch the process is a pain. The reason is that the emblem area seems to be either insensitive or too small to hit very easily, and far too easy to click the file/folder beneath and have that launch/open.
I operate with lots of folders and very large image files (over 150mb), accidentally clicking one when trying to select a bunch of things for moving/deletion brings eye of gnome out of the depths after a minute or two, leaving me waiting to simply close it and a return of functionality.
Resorting to ctrl/shift + left-click is a bit pointless and infuriating when the aforementioned feature is available, but unusable. This is enough for me to prefer Nautilus regardless of current development state for Marlin, just saying.
This bug is related to thumbnails. The click zone computations need to be updated. Wow u prefer a stable application vs a non released and unstable one, what a scoop!!!