1636969 - Selecting text on view-source:https://hg.mozilla.org/users/ is extremely slow

Status: NEW

(Core :: Graphics: WebRender, defect, P3)

Description

[:glob ✱]

Seen on 78.0a1 (2020-05-11) (64-bit) on MacOS, Windows, and Linux.

Steps to reproduce:

1. Visit https://hg.mozilla.org/users/ (81kb)
2. View Source
3. Select some text

Expected outcome:

• Text is immediately displayed as selected

Actual outcome:

• Selection is sluggish; sometimes taking several seconds to update.

Occurs with webrender enabled and disabled.
Accessibility features are disabled.

Comment 1

[Ted Campbell [:tcampbell]]

I see 100% WebRender time when I profile on Windows 10 : https://perfht.ml/2Aau4Vl

Comment 2

[:glob ✱]

I can no longer reproduce this issue.

Comment 3

[A.D.F.]

Problem of lethargic slowness in selecting large portions of text by keyboard or by mouse in "View source" window continues to be hundreds or thousands of times slower than the same operation in other browsers (Chrome, etc.).

For the sake of clarity I report a test case that can be reproduced and noticed also in modern PC with fast CPUs.

PC CPU 3.7 Ghz, 6 GB RAM, Windows 10 64 bit.
Firefox 94.0.1 (64 bit).

Steps to reproduce

1. Browse a big web page with more than 100kB of HTML code and long lines, e.g. https://hg.mozilla.org/users/
   or even better an over 1MB HTML page like https://en.wikipedia.org/wiki/Border_control.
2. Mouse click on page, right mouse button, View Source.
3. Mouse click on the beginning of view source text, then select text by dragging down mouse pointer or even better (to see the problem), press keyboard keys SHIFT+PgDn to select and scroll down text.

Actual results

Text is selected but the operation gets exponentially slower as soon as selected size of text increases.
After 3..4 seconds increasing slowness in text selection becomes absolutely unacceptable.

Expected results.

Selecting text in view source should be a fast operation (just like it was several years ago).

In Chrome and other browsers selecting text by mouse in "View source" is really fast (not as fast as it could be but certainly it is 100 .. 1000 times faster than Firefox) and above all operation does not slow down as size of selected text increases.

Comment 4

[A.D.F.]

No offence is intended, but I would like to add that this and other defects in Firefox are a bit too evident to everyday users; it looks like they belong more to some ancient FF development betas of 10 .. 12 years ago than nowadays.

Comment 5

[:glob ✱]

I'm still unable to reproduce this issue any more on macOS; might be Windows specific now given comment 3.

Comment 6

[Glenn Watson [:gw]]

I can reproduce this (somewhat) on Linux. I see high times in scene building, gecko DL and WR DL profiling buckets. This is likely due to how we currently need to rebuild all those text runs each scene build.

Comment 7

[Chauncey McAskill]

I can reproduce this on FF 107.0b2 (macOS 12.6, M1 2020). Slow view-source started a few months ago.

Related:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1282552