User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:77.0) Gecko/20100101 Firefox/77.0

Steps to reproduce:

1. Use Windows 10 with a dual monitor setup. Have the primary monitor set on the left hand side of the secondary monitor.
2. Right-click on the desktop and select 'Display settings'. In the Display settings screen, set the scale to 125%.
3. Open Firefox to a scrollable website.
4. Move the window onto the secondary monitor's screen, towards the left-hand side.
5. Middle-click the mouse close to the left edge of the screen.

Actual results:

The icon that signifies that you have entered into middle-click-scrolling mode appears on the very right edge of the primary monitor, rather than on the secondary monitor where the cursor was positioned.

My screen resolution is set to 2560 x 1440 px.
Windows is using a screen scale value of 125%.
The bug only seems to occur if you click within 640 pixels of the edge of the screen.
Note that 640 is 25% of 2560.

Expected results:

The middle-click-scroll icon should always appear directly under the cursor when the middle mouse-button is clicked.

The severity field is not set for this bug.
:jimm, could you have a look please?

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Hello,

I have successfully managed to reproduce the issue using the provided steps and a similar setup (main screen: 2560 x 1440 with 125% scale, second screen: 1920 x 1080 with 125% scale), on the latest Firefox Nightly 86.0a1.
Setting the issue to new. Severity and priority won't change as the issue is very particular to the setup and the position of the cursor when the middle click occurs.

Hey Tim, could you find some time to poke at this?

I was able to reproduce - scrolling works as expected, but the scroll inidcator is in the wrong place. Both screens were at 125%. Interesting note, when I had the first screen at 125%, and the second at 100%, I could not reproduce.

I can reproduce this issue with two 3840 x 2160 monitors side by side horizontally.

• The issue isn't present when both monitors are at 100% scaling.
• When both monitors are at 125% scaling, pressing in the left 25% of the right monitor reproduces this issue.
• When both monitors are at 150% scaling, pressing in the left 50% of the right monitor reproduces this issue.

This was fixed in Firefox 98 in bug 1474783, about a year ago.