I recently upgraded to Emacs 25.1.2, and
transient-mark-mode no longer works properly - sometimes.
The behavior I see is that pressing
C-spc and moving the cursor to select an area of text doesn't automatically highlight the area. If I move to another window and return, or switch to a different window and come back, I see the selection highlighted.
I also see the section highlighted if the cursor comes across a parenthesis. Since I have show-paren-mode turned on, the parens get highlighted, and the selection also gets highlighted.
It's suddenly working as expected as I type this, but it wasn't working for the past couple of days, in spite of restarts.
How do I debug what might be happening?