Update: I did manage to get the page to scroll, but it's super flaky.

This only repros with very high scale factor. If you need a temp workaround, ctrl-shift-0 will return you to baseline. 

Looks like we transform AppsGridView but do not use the transformed bounds in the calculation to determine whether the app icon is in the drag buffer.

Fix for review:
https://chromium-review.googlesource.com/c/chromium/src/+/1354479/

Bulk moving <p-1's to the next milestone because we branched to M-73.

(didn't mean to grab these P-1's)

This will involve some refactoring to fix properly. I'll try to land and merge my short fix, then refactor in M-73.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9504f17bdd7940d0d8064259f57291baa895002e

commit 9504f17bdd7940d0d8064259f57291baa895002e
Author: Alex Newcomer <newcomer@chromium.org>
Date: Thu Dec 06 18:49:10 2018

cros: Update AppsGridView bottom drag buffer

The AppsGridView is transformed to fit on screens that use display zoom.

Use the current transform to calculate the bottom of the AppsGridView,
then consider all drags below AppsGridView "in the buffer". The old
buffer values do not make sense with the redesign, and with a scaled
display.

Bug:  907914 
Change-Id: I40e15fc8ba4f1022a9def18ac97fd0bab0d2d031
Reviewed-on: https://chromium-review.googlesource.com/c/1354479
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614429}
[modify] https://crrev.com/9504f17bdd7940d0d8064259f57291baa895002e/ash/app_list/views/apps_grid_view.cc
[modify] https://crrev.com/9504f17bdd7940d0d8064259f57291baa895002e/ash/app_list/views/apps_grid_view_unittest.cc

Requesting merge of the quick fix, in the mean-time I will get to work on refactoring.

I talked to weidongg@ about this, he says we implemented it this way because without transforms, the grid was jittering due to rounding errors. Was the jittering seen on an actual device, or on the emulator?

Whenever I change the UI scale, the laucher closes. So maybe it's only a problem on the emulator (when resizing the window with a mouse).

Re #13, what I tried is scale down the apps grid by divide all integer size by the scale (double) and then convert to integer size again. Is there any other way that we can avoid the rounding error?

Transform is introduced in https://chromium-review.googlesource.com/c/1259463. The intention seems to ensure the grid's visual size is bigger than  AppsGridView::GetMinimumTileGridSize, rather than rounding errors.

Can we fix the sizing issue by other means? e.g. use a smaller size for icon and decrease the padding between icon and title. Setting transform on a view indefinitely is a bit unusual and we might have similar issues in the future.

Your change meets the bar and is auto-approved for M72. Please go ahead and merge the CL to branch 3626 manually. Please contact milestone owner if you have questions.
Owners: govind@(Android), kariahda@(iOS), djmm@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/86e7b07e2e60777f2ee57dffd857f664b5a38211

commit 86e7b07e2e60777f2ee57dffd857f664b5a38211
Author: Alex Newcomer <newcomer@chromium.org>
Date: Mon Dec 10 22:17:32 2018

cros: Update AppsGridView bottom drag buffer

The AppsGridView is transformed to fit on screens that use display zoom.

Use the current transform to calculate the bottom of the AppsGridView,
then consider all drags below AppsGridView "in the buffer". The old
buffer values do not make sense with the redesign, and with a scaled
display.

Bug:  907914 
Change-Id: I40e15fc8ba4f1022a9def18ac97fd0bab0d2d031
Reviewed-on: https://chromium-review.googlesource.com/c/1354479
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#614429}(cherry picked from commit 9504f17bdd7940d0d8064259f57291baa895002e)
Reviewed-on: https://chromium-review.googlesource.com/c/1370722
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/branch-heads/3626@{#237}
Cr-Branched-From: d897fb137fbaaa9355c0c93124cc048824eb1e65-refs/heads/master@{#612437}
[modify] https://crrev.com/86e7b07e2e60777f2ee57dffd857f664b5a38211/ash/app_list/views/apps_grid_view.cc
[modify] https://crrev.com/86e7b07e2e60777f2ee57dffd857f664b5a38211/ash/app_list/views/apps_grid_view_unittest.cc

Since the quick fix was merged, to avoid confusion with follow up CLs, I moved the follow up work to  issue 914509 

The following revision refers to this bug: 
https://chromium.googlesource.com/chromium/src.git/+/86e7b07e2e60777f2ee57dffd857f664b5a38211

Commit: 86e7b07e2e60777f2ee57dffd857f664b5a38211
Author: newcomer@chromium.org
Commiter: newcomer@chromium.org
Date: 2018-12-10 22:17:32 +0000 UTC

cros: Update AppsGridView bottom drag buffer

The AppsGridView is transformed to fit on screens that use display zoom.

Use the current transform to calculate the bottom of the AppsGridView,
then consider all drags below AppsGridView "in the buffer". The old
buffer values do not make sense with the redesign, and with a scaled
display.

Bug:  907914 
Change-Id: I40e15fc8ba4f1022a9def18ac97fd0bab0d2d031
Reviewed-on: https://chromium-review.googlesource.com/c/1354479
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#614429}(cherry picked from commit 9504f17bdd7940d0d8064259f57291baa895002e)
Reviewed-on: https://chromium-review.googlesource.com/c/1370722
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/branch-heads/3626@{#237}
Cr-Branched-From: d897fb137fbaaa9355c0c93124cc048824eb1e65-refs/heads/master@{#612437}