376578 - drag feedback is offset from cursor location when css pixels are scaled

Status: RESOLVED FIXED

(Core :: DOM: Copy & Paste and Drag & Drop, defect, P4)

Description

[Daniel Brooks [:db48x]]

When I set the layout.css.dpi pref to 150 to make Firefox scale all css pixels to 2 device pixels, any drag feedback is offset from the correct cursor location. I haven't measured it, but it looks very much like the coordinates of the feedback are scaled by the same amount as the scaling caused by setting the pref.

During a discussion in #extdev, we were looking at http://mxr-test.landfill.bugzilla.org/mozilla/source/content/events/src/nsDOMUIEvent.cpp#116, which looks to me like it should return device pixels, not css pixels. On the other hand, that seems like it would cause the feedback to be offset in the other direction, so it could be unrelated. I haven't looked at how the feedback stuff works either.

Comment 1

[Daniel Brooks [:db48x]]

Another thing, the offset changes if it scales the feedback. It may be scaling the offset by the same amount as it scales the feedback.

Comment 2

[Neil Deakin]

Doesn't seem to be a problem on Mac.

Comment 3

[Boris Zbarsky [:bzbarsky]]

Is this still a problem?  And is it also a problem on full zoom?

Comment 4

[Stuart Parmenter]

re-nom if this is still an issue.  i'm not sure you really want to be changing that dpi setting.

Comment 5

[Boris Zbarsky [:bzbarsky]]

The DPI setting just mimicks the effects of actually having that display resolution, so any problem it causes would also happen on high-res displays...

Comment 6

[Daniel Brooks [:db48x]]

This is definitely a regression due to the high-dpi scaling. Full page zoom has no effect on it, it only happens when the dpi is enough to cause the scaling (whether that's because you changed the pref, or because gecko inherited the dpi from the OS, which is how most people will stumble onto the problem.)

Comment 7

[Damon Sicore (:damons)]

+'ing and setting as a P4.  

Comment 8

[Neil Deakin]

Attachment: convert screen coordinates into unscaled device pixels

Comment 9

[Robert O'Callahan (:roc) (email my personal email if necessary)]

+  if (aDOMNode) {

Make this an early return if (!aDOMNode)

+    if (document) {
+      nsIPresShell* presShell = document->GetPrimaryShell();
+      if (presShell) {

Early returns here too

I think that ConvertToUnscaledDevPixels should take a presshell or prescontext parameter. Ideally we would pass prescontexts/presshells around instead of getting them from the node, or at least get them from the node just once per drag API call.

Comment 10

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Not for performance reasons, just because of code size and also flexibility so we're not depending on "one presshell per document" too much.

Comment 11

[Neil Deakin]

Attachment: address comments