User Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:20.0) Gecko/20121213 Firefox/20.0 Build ID: 20121213030751 Steps to reproduce: 1. Set layout.css.devPixelsPerPx to -1 2. Go to the Control Panel and set the DPI to 150% 3. Open at least two tabs 4. Use the mouse to drag the tab somewhere Actual results: The thumbnail that appears when dragging the tab is in the wrong position. It is not next to the cursor. This problem is more noticeable if the tabs are on the right side of the window.

When creating the drag image, we need to convert the mouse coordinates from global display pixels to the context's device pixels for offsetting the drag image rect from the click position. The Mac HiDPI patches in bug 674373 did this in widget/cocoa/nsDragService.mm before calling DrawDrag. We could do a similar thing in widget/windows/nsDragService.cpp, but it seems more sensible to define the aScreenX/Y parameters of nsBaseDragService::DrawDrag as being display pixels, and handle the conversion to device pix there rather than duplicating it in each platform-specific file.

Tryserver build: http://ftp-scl3.mozilla.com/pub/mozilla.org/firefox/try-builds/jkew@mozilla.com-61c14d9b87c8/try-win32/ (note that it does -not- include the patch for bug 821454, so that problem will still be present).

(In reply to Jonathan Kew (:jfkthame) from comment #2) > Tryserver build: > http://ftp-scl3.mozilla.com/pub/mozilla.org/firefox/try-builds/jkew@mozilla. > com-61c14d9b87c8/try-win32/ (note that it does -not- include the patch for > bug 821454, so that problem will still be present). Yeah, it works on my PC. :)