--- a/dom/base/test/test_bug320799.html
+++ b/dom/base/test/test_bug320799.html
@@ -47,23 +47,28 @@ https://bugzilla.mozilla.org/show_bug.cg
     <option>x</option>
     <option>x</option>
     <option>x</option>
     <option>x</option>
   </select>
   <select id="s3">
     <option>x</option>
   </select>
+  <select id="s4" style="width: 100px; box-sizing: border-box; border: 0; margin: 10px">
+    <option>This is a test, it really is a test I tell you</option>
+  </select>
 </p>
 <div id="content" style="display: none">
   
 </div>
 <pre id="test">
 <script type="application/javascript">
 
 /** Test for Bug 320799 **/
 is($("s").scrollWidth, 100, "Scroll width should not include dropdown contents");
 is($("s2").clientWidth, $("s3").clientWidth,
    "Client width should not depend on the dropdown's vertical scrollbar");
+
+is($("s4").scrollWidth, 100, "Scroll width should not include dropdown contents");
 </script>
 </pre>
 </body>
 </html>

--- a/layout/forms/nsComboboxControlFrame.cpp
+++ b/layout/forms/nsComboboxControlFrame.cpp
@@ -223,16 +223,17 @@ static int32_t gReflowInx = -1;
 nsComboboxControlFrame::nsComboboxControlFrame(ComputedStyle* aStyle,
                                                nsPresContext* aPresContext)
     : nsBlockFrame(aStyle, aPresContext, kClassID),
       mDisplayFrame(nullptr),
       mButtonFrame(nullptr),
       mDropdownFrame(nullptr),
       mListControlFrame(nullptr),
       mDisplayISize(0),
+      mMaxDisplayISize(0),
       mRecentSelectedIndex(NS_SKIP_NOTIFY_INDEX),
       mDisplayedIndex(-1),
       mLastDropDownBeforeScreenBCoord(nscoord_MIN),
       mLastDropDownAfterScreenBCoord(nscoord_MIN),
       mDroppedDown(false),
       mInRedisplayText(false),
       mDelayedShowDropDown(false),
       mIsOpenInParentProcess(false){REFLOW_COUNTER_INIT()}
@@ -823,16 +824,19 @@ void nsComboboxControlFrame::Reflow(nsPr
         PresContext(), aReflowInput.mRenderingContext, wm);
     if (buttonISize > aReflowInput.ComputedISize()) {
       buttonISize = 0;
     }
   }
 
   mDisplayISize = aReflowInput.ComputedISize() - buttonISize;
 
+  mMaxDisplayISize =
+      mDisplayISize + aReflowInput.ComputedLogicalPadding().IEnd(wm);
+
   nsBlockFrame::Reflow(aPresContext, aDesiredSize, aReflowInput, aStatus);
 
   // The button should occupy the same space as a scrollbar
   nsSize containerSize = aDesiredSize.PhysicalSize();
   LogicalRect buttonRect = mButtonFrame->GetLogicalRect(containerSize);
 
   buttonRect.IStart(wm) =
       aReflowInput.ComputedLogicalBorderPadding().IStartEnd(wm) +
@@ -1240,22 +1244,34 @@ void nsComboboxDisplayFrame::Reflow(nsPr
     // (This frame has 'line-height: -moz-block-height' in the UA
     // sheet which is suitable when there's a specified block-size.)
     auto lh = ReflowInput::CalcLineHeight(mComboBox->GetContent(),
                                           mComboBox->Style(), aPresContext,
                                           NS_UNCONSTRAINEDSIZE, inflation);
     state.SetComputedBSize(lh);
   }
   WritingMode wm = aReflowInput.GetWritingMode();
-  nscoord computedISize = mComboBox->mDisplayISize -
-                          state.ComputedLogicalBorderPadding().IStartEnd(wm);
-  if (computedISize < 0) {
-    computedISize = 0;
+  nscoord inlineBp = state.ComputedLogicalBorderPadding().IStartEnd(wm);
+  nscoord computedISize = mComboBox->mDisplayISize - inlineBp;
+
+  // Other UAs ignore padding in some (but not all) platforms for (themed only)
+  // comboboxes. Instead of doing that, we prevent that padding if present from
+  // clipping the display text, by enforcing the display text minimum size in
+  // that situation.
+  const bool shouldHonorMinISize =
+      mComboBox->StyleDisplay()->mAppearance == StyleAppearance::Menulist;
+  if (shouldHonorMinISize) {
+    computedISize = std::max(state.ComputedMinISize(), computedISize);
+    // Don't let this size go over mMaxDisplayISize, since that'd be
+    // observable via clientWidth / scrollWidth.
+    computedISize =
+        std::min(computedISize, mComboBox->mMaxDisplayISize - inlineBp);
   }
-  state.SetComputedISize(computedISize);
+
+  state.SetComputedISize(std::max(0, computedISize));
   nsBlockFrame::Reflow(aPresContext, aDesiredSize, state, aStatus);
   aStatus.Reset();  // this type of frame can't be split
 }
 
 void nsComboboxDisplayFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
                                               const nsDisplayListSet& aLists) {
   nsDisplayListCollection set(aBuilder);
   nsBlockFrame::BuildDisplayList(aBuilder, set);

--- a/layout/forms/nsComboboxControlFrame.h
+++ b/layout/forms/nsComboboxControlFrame.h
@@ -284,16 +284,21 @@ class nsComboboxControlFrame final : pub
   nsContainerFrame* mDisplayFrame;     // frame to display selection
   nsIFrame* mButtonFrame;              // button frame
   nsIFrame* mDropdownFrame;            // dropdown list frame
   nsListControlFrame* mListControlFrame;  // ListControl for the dropdown frame
 
   // The inline size of our display area.  Used by that frame's reflow
   // to size to the full inline size except the drop-marker.
   nscoord mDisplayISize;
+  // The maximum inline size of our display area, which is the
+  // nsComoboxControlFrame's border-box.
+  //
+  // Going over this would be observable via DOM APIs like client / scrollWidth.
+  nscoord mMaxDisplayISize;
 
   nsRevocableEventPtr<RedisplayTextEvent> mRedisplayTextEvent;
 
   int32_t mRecentSelectedIndex;
   int32_t mDisplayedIndex;
   nsString mDisplayedOptionTextOrPreview;
 
   // make someone to listen to the button. If its programmatically pressed by

--- a/layout/generic/ReflowInput.h
+++ b/layout/generic/ReflowInput.h
@@ -624,17 +624,18 @@ struct ReflowInput : public SizeComputat
   nscoord mComputedHeight;
 
   // Computed values for 'left/top/right/bottom' offsets. Only applies to
   // 'positioned' elements. These are PHYSICAL coordinates (for now).
   nsMargin mComputedOffsets;
 
   // Computed values for 'min-width/max-width' and 'min-height/max-height'
   // XXXldb The width ones here should go; they should be needed only
-  // internally.
+  // internally, except for nsComboboxDisplayFrame, which still wants to honor
+  // min-inline-size even though it wants to trump inline-size.
   MOZ_INIT_OUTSIDE_CTOR
   nscoord mComputedMinWidth, mComputedMaxWidth;
   MOZ_INIT_OUTSIDE_CTOR
   nscoord mComputedMinHeight, mComputedMaxHeight;
 
  public:
   // Our saved containing block dimensions.
   LogicalSize mContainingBlockSize = LogicalSize(mWritingMode);

--- a/layout/reftests/forms/select/reftest.list
+++ b/layout/reftests/forms/select/reftest.list
@@ -9,8 +9,13 @@ fuzzy(0-1,0-4) == padding-button-placeme
 fuzzy-if(skiaContent,0-4,0-1) needs-focus == focusring-1.html focusring-1-ref.html
 needs-focus == focusring-2.html focusring-2-ref.html
 needs-focus == focusring-3.html focusring-3-ref.html
 == dynamic-text-indent-1.html dynamic-text-indent-1-ref.html
 == dynamic-text-overflow-1.html dynamic-text-overflow-1-ref.html
 == listbox-zero-row-initial.html listbox-zero-row-initial-ref.html
 == 1246836.html 1246836-ref.html
 skip-if(Android) == select-option-display-none-inline-size.html select-option-display-none-inline-size-ref.html
+
+# Android and Windows actually use the anonymous select > button (rather than
+# drawing the arrow as a background like Linux and Mac), so most of this test
+# doesn't apply since when over-constrained it gets zero-sized.
+skip-if(Android||winWidget) == themed-select-padding-no-clip.html themed-select-padding-no-clip-ref.html

new file mode 100644
--- /dev/null
+++ b/layout/reftests/forms/select/themed-select-padding-no-clip-ref.html
@@ -0,0 +1,37 @@
+<!doctype html>
+<title>Bug 1561794 - padding-inline-end does not clip display text for themed comboboxes.</title>
+<style>
+  select, .unthemed {
+    font: 16px / 1 monospace;
+    width: 20ch;
+    display: block;
+  }
+  .unthemed {
+    box-sizing: border-box;
+    border: 1px solid grey;
+  }
+</style>
+<!--- Ensure the text isn't chopped -->
+<select><option>XXXXXXXXXXXX</option></select>
+<select><option>X XXXXXXXXXX</option></select>
+
+<!--- Test that the display text doesn't overflow the select. -->
+<select style="overflow: hidden"><option>XXXXXXXXXXXXXXXXX</option></select>
+
+<!-- Test that this only affects themed comboboxes -->
+<div class="unthemed" style="padding-right: 15ch"><div style="padding: 1px 4px; overflow: hidden;">XXXXXXXXXX</div></div>
+
+<select id="src" style="padding-left: 10ch; padding-right: 10ch"><option>XXXXXXXXXX</option></select>
+<select id="ref" style="padding-left: 10ch;"><option>XXXXXXXXXX</option></select>
+<script>
+  // This one is a bit special, because we want `ref` to have the same size as
+  // `src`, which for some reason, with or without the patch for this bug,
+  // grows outside the specified width. Given this width depends on scrollbar,
+  // width, which is platform dependent, we just calculate it from `src`, and
+  // then hide it.
+  //
+  // We just really want to test that the padding right doesn't clip the text
+  // anyhow.
+  ref.style.width = getComputedStyle(src).width;
+  src.style.display = "none";
+</script>

new file mode 100644
--- /dev/null
+++ b/layout/reftests/forms/select/themed-select-padding-no-clip.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>Bug 1561794 - padding-inline-end does not clip display text for themed comboboxes.</title>
+<style>
+  select {
+    font: 16px / 1 monospace;
+    width: 20ch;
+    display: block;
+  }
+  .unthemed {
+    -moz-appearance: none;
+    -webkit-appearance: none;
+    border: 1px solid grey;
+    background: transparent;
+  }
+</style>
+<!--- Ensure the text isn't chopped -->
+<select style="padding-right: 15ch"><option>XXXXXXXXXXXX</option></select>
+<select style="padding-right: 15ch"><option>X XXXXXXXXXX</option></select>
+
+<!--- Test that the display text doesn't overflow the select. It'd be nice to test it doesn't overlap the button (which is true), but that seems hard to test via a reftest -->
+<select style="padding-right: 15ch"><option>XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</option></select>
+
+<!-- Test that this only affects themed comboboxes; 4px is the padding-inline-start of the comboboxcontrol-frame anon box, so as to clip exactly at a character boundary. -->
+<select class="unthemed" style="color: initial; padding-right: 15ch"><option>XXXXXXXXXX</option></select>
+
+<select style="padding-left: 10ch; padding-right: 10ch"><option>XXXXXXXXXX</option></select>

--- a/layout/style/res/forms.css
+++ b/layout/style/res/forms.css
@@ -329,18 +329,20 @@ select > button[orientation="right"]:act
   padding-block-end: 1px;
   padding-inline-start: 4px;
   padding-inline-end: 4px;
   color: unset;
   white-space: nowrap;
   text-align: unset;
   user-select: none;
   /* Make sure to size correctly if the combobox has a non-auto block-size. */
-  block-size: 100% ! important;
-  box-sizing: border-box ! important;
+  block-size: 100%;
+  /* Try to always display our own text */
+  min-inline-size: max-content;
+  box-sizing: border-box;
   line-height: -moz-block-height;
 }
 
 option {
   display: block;
   float: none !important;
   position: static !important;
   min-block-size: 1em;