Auto merge of #29863 - nicoburns:fix-infinite-loop-when-shrinking, r=Loirooriol

Fix infinite loop in flexbox algorithm

Only apply step 5c of "resolve flexible lengths" if sum of scaled flexible shrink factors > 0
Probably fixes #29852 (but speculative as I can't get mach to run).

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

components/layout_2020/flexbox/layout.rs

@@ -976,6 +976,7 @@ impl FlexLine<'_> {
                     let scaled_shrink_factors_sum: Length = unfrozen_items()
                         .map(|(item, _)| scaled_shrink_factor(item))
                         .sum();
+                    if scaled_shrink_factors_sum > Length::zero() {
                         for (item, target_main_size) in unfrozen_items() {
                             let ratio = scaled_shrink_factor(item) / scaled_shrink_factors_sum;
                             target_main_size
@@ -983,6 +984,7 @@ impl FlexLine<'_> {
                         }
                     }
                 }
+            }
 
             // “Fix min/max violations.”
             let violation = |(item, target_main_size): (&FlexItem, &Cell<Length>)| {

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-aspect-ratio-img-row-013.html.ini

@@ -1,5 +1,4 @@
 [flex-aspect-ratio-img-row-013.html]
-  expected: TIMEOUT
   [img 1]
     expected: FAIL
 

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-item-compressible-002.html.ini

@@ -1,5 +1,4 @@
 [flex-item-compressible-002.html]
-  expected: TIMEOUT
   [.flexbox 14]
     expected: FAIL
 

tests/wpt/metadata-layout-2020/css/css-flexbox/flex-minimum-height-flex-items-031.html.ini

@@ -1,5 +1,4 @@
 [flex-minimum-height-flex-items-031.html]
-  expected: TIMEOUT
   [.flex 1]
     expected: FAIL
 

tests/wpt/metadata-layout-2020/css/css-flexbox/flexbox-align-self-stretch-vert-001.html.ini

@@ -1,2 +1,2 @@
 [flexbox-align-self-stretch-vert-001.html]
-  expected: TIMEOUT
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/flexbox-overflow-vert-002.html.ini

@@ -1,2 +1,2 @@
 [flexbox-overflow-vert-002.html]
-  expected: TIMEOUT
+  expected: FAIL

tests/wpt/metadata-layout-2020/css/css-flexbox/percentage-padding-002.html.ini

@@ -1,2 +1,2 @@
 [percentage-padding-002.html]
-  expected: TIMEOUT
+  expected: FAIL