Mouse button thinks it's being held down the whole time a preview is being rendered in Backgrounds: Open dialog.

[jstressman]

I'm not sure which component this should be filed under, so I'm just adding it under Background preferences for now. (Not sure if it's a more basic issue with mouse click handling, or with the addition of image previews in the Backgrounds prefs specifically.)

I'm copying the description of this from #8778 so that it doesn't get lost in that ticket.

When you select a large image in the Open dialog, a bug is triggered in which the delay in loading the preview apparently causes the dialog to think that the mouse is being held down the entire time, and so when the preview finishes rendering and the file apparently "finishes" being selected, thinking you've been holding the mouse button down the whole time... so the context menu pops up when you don't want it to.

This bug shows up on a number of my wallpapers; anything over around 4 or 5MB for me. Which is about 9 of the wallpapers in my wallpapers folder. Since this appears to be related to the time it takes to load the preview, this might be much more noticeable on slower systems, being triggered by smaller filesizes.

hrev44404 gcc2h

[jstressman]

This is still present in hrev45308.

(this bug was also referenced in #8714)

I had some more thoughts on it as well. It would seem preferable if the click event finished before the preview started rendering, or at least happened in a different thread or something? So that if we choose to click on a different file before that one finishes rendering, we can move on with that second file instead of leaving the list locked up until the previous image finishes, which just adds frustration to having the context menu pop up when it finishes... which sometimes also leads to other strange behavior like it continuing to think the mouse button is still down and drawing selection boxes when you move the mouse... so you have to click again somewhere to get things back to normal.