It seems that selection merging gives different results based on how a selection was made exactly. Here are two scenarios which give different results, even though the selections involved are the same.

First, I double click to select a word, then cmd-click-draw to create another selection and merge it with the first one:

Notice that the selection which survives the merging is the one I created first.

Second, I click-draw to select a word, then cmd-click-draw to create another selection and merge it with the first one:

Notice that the selection which survives the merging is the one I created second.

In the first case, I expected that the selection which I was currently dragging would survive the merging (as in the second case).

cc @benogle and @nathansobo (who was recently looking at how merging works)