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Middle click paste triggers when it shouldn’t
I use Firefox on X11/GNU/Linux. The X11 convention is to paste the selected text into a text box on a middle click.
I also use the autoscrolling feature in Firefox (Preferences → General → Browsing → [x] Use autoscrolling. With this, a middle click in a page displays a circle at the mouse position, and moving mouse away from this circle initiates scrolling in the corresponding direction. Alternatively, holding down the middle button and moving it around also works.
It is obvious to me that middle-click paste should only trigger if I click in an editable text box such as an <input>, <textarea>, or any element with the contenteditable attribute set to true.
However, on some sites, I find that selected text is pasted when I intend to scroll. On Reddit, it happens when I scroll and release the middle button in the comment entry text box. On Slack, after their today’s update, it happens any time I middle-click on anything that does not handle middle clicks on its own (e.g. links).
Slack support says they can’t test or debug on X11/GNU/Linux.
I tried making a simple page with a contenteditable element in it. It does not exhibit this issue. Text is pasted if and only if I start and end the click inside the text box.
How do I debug it further?