Auto merge of #12754 - stshine:no-divide-by-zero, r=pcwalton

Fix unexpected freeze of flex item

<!-- Please describe your changes on the following line: -->

Fix the currently logic that a item will freeze if it should
grow(shrink) and its basesize is less(more) than its min(max) size. This
also fix the divide by zero error when an item should shrink but it has
zero length and zero min size.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

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components/layout/flex.rs

@@ -276,9 +276,8 @@ impl FlexLine {
         // https://drafts.csswg.org/css-flexbox/#resolve-flexible-lengths
         for item in items.iter_mut().filter(|i| !(i.is_strut && collapse)) {
             item.main_size = max(item.min_size, min(item.base_size, item.max_size));
-            if item.main_size != item.base_size
-                || (self.free_space > Au(0) && item.flex_grow == 0.0)
-                || (self.free_space < Au(0) && item.flex_shrink == 0.0) {
+            if (self.free_space > Au(0) && (item.flex_grow == 0.0 || item.base_size >= item.max_size)) ||
+                (self.free_space < Au(0) && (item.flex_shrink == 0.0 || item.base_size <= item.min_size)) {
                     item.is_frozen = true;
                 } else {
                     item.is_frozen = false;
@@ -311,7 +310,7 @@ impl FlexLine {
                     (item.flex_shrink * item.base_size.0 as f32 / total_scaled, item.min_size)
                 };
                 let variation = self.free_space.scale_by(factor);
-                if variation.0.abs() > (end_size - item.main_size).0.abs() {
+                if variation.0.abs() >= (end_size - item.main_size).0.abs() {
                     // Use constraint as the target main size, and freeze item.
                     total_variation += end_size - item.main_size;
                     item.main_size = end_size;

tests/wpt/metadata-css/css-flexbox-1_dev/html/flexbox_computedstyle_min-width-auto.htm.ini

@@ -1,4 +1,5 @@
 [flexbox_computedstyle_min-width-auto.htm]
   type: testharness
-  expected: TIMEOUT
+  [flexbox | computed style | min-width: auto]
+    expected: FAIL