When the user clicks on the combobox it sets an attribute that causes CSS to change the style attr of the dropdown from collapsed to visible. This generates an Incremental reflow on the combobox dropdown. This is fine. But for some reason the block containing the comboxbox thinks its dirty and that it needs to be reflowed. At times this can be expensive. The easiest thing to do is to load the attached example, then set a break point in the AppendReflowCommand in the PresShell. The first stop is fine the second stop is uneeded. Note that the block frame that thinks it has dirty children is 0x0117f8ec, and it is the frame that contains the combobox. Here is the call stack at that point: PresShell::AppendReflowCommand(PresShell * const 0x024e5230, nsIReflowCommand * 0x0155d1d0) line 1890 nsFrame::CreateAndPostReflowCommand(nsIPresShell * 0x024e5230, nsIFrame * 0x0117f8ec, nsIReflowCommand::ReflowType ReflowDirty, nsIFrame * 0x00000000, nsIAtom * 0x00000000 {???}, nsIAtom * 0x00000000 {???}) line 2738 nsBlockFrame::ReflowDirtyChild(nsBlockFrame * const 0x0117f8ec, nsIPresShell * 0x024e5230, nsIFrame * 0x00000000) line 6192 + 21 bytes nsBlockFrame::Reflow(nsBlockFrame * const 0x0117f8ec, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 1559 + 34 bytes nsBlockReflowContext::ReflowBlock(nsIFrame * 0x0117f8ec, const nsRect & {x=0 y=0 width=13380 height=1073741824}, int 1, int 0, int 1, nsMargin & {top=0 right=0 bottom=0 left=0}, unsigned int & 0) line 449 + 45 bytes nsBlockFrame::ReflowBlockFrame(nsBlockReflowState & {...}, nsLineBox * 0x024fc8e0, int * 0x0012ea44) line 3550 + 59 bytes nsBlockFrame::ReflowLine(nsBlockReflowState & {...}, nsLineBox * 0x024fc8e0, int * 0x0012ea44, int 1) line 2863 + 23 bytes nsBlockFrame::ReflowDirtyLines(nsBlockReflowState & {...}) line 2670 + 27 bytes nsBlockFrame::Reflow(nsBlockFrame * const 0x0117f864, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 1589 + 15 bytes nsAreaFrame::Reflow(nsAreaFrame * const 0x0117f864, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 272 + 25 bytes nsContainerFrame::ReflowChild(nsIFrame * 0x0117f864, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, int 0, int 0, unsigned int 0, unsigned int & 0) line 646 + 31 bytes RootFrame::Reflow(RootFrame * const 0x0117f7ec, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 331 nsContainerFrame::ReflowChild(nsIFrame * 0x0117f7ec, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, int 0, int 0, unsigned int 1, unsigned int & 0) line 646 + 31 bytes nsScrollFrame::Reflow(nsScrollFrame * const 0x0117f828, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 680 nsContainerFrame::ReflowChild(nsIFrame * 0x0117f828, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, int 0, int 0, unsigned int 0, unsigned int & 0) line 646 + 31 bytes ViewportFrame::Reflow(ViewportFrame * const 0x0117f7b0, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsHTMLReflowState & {...}, unsigned int & 0) line 531 nsHTMLReflowCommand::Dispatch(nsHTMLReflowCommand * const 0x0155a030, nsIPresContext * 0x02478600, nsHTMLReflowMetrics & {...}, const nsSize & {width=13380 height=2835}, nsIRenderingContext & {...}) line 145 PresShell::ProcessReflowCommands(PresShell * const 0x024e5230, int 0) line 2056 PresShell::FlushPendingNotifications(PresShell * const 0x024e5230) line 2520 nsComboboxControlFrame::ShowList(nsIPresContext * 0x02478600, int 1) line 539 nsComboboxControlFrame::ToggleList(nsComboboxControlFrame * const 0x0117faa4, nsIPresContext * 0x02478600) line 1863 nsComboboxControlFrame::ShowDropDown(nsComboboxControlFrame * const 0x0117faf4, int 1) line 1797 nsListControlFrame::MouseDown(nsIDOMEvent * 0x0153df24) line 2845 nsEventListenerManager::HandleEvent(nsIPresContext * 0x02478600, nsEvent * 0x0012fc1c, nsIDOMEvent * * 0x0012f8d4, unsigned int 7, nsEventStatus * 0x0012fb28) line 758 + 17 bytes nsGenericElement::HandleDOMEvent(nsIPresContext * 0x02478600, nsEvent * 0x0012fc1c, nsIDOMEvent * * 0x0012f8d4, unsigned int 1, nsEventStatus * 0x0012fb28) line 1010 nsHTMLSelectElement::HandleDOMEvent(nsHTMLSelectElement * const 0x02507b44, nsIPresContext * 0x02478600, nsEvent * 0x0012fc1c, nsIDOMEvent * * 0x00000000, unsigned int 1, nsEventStatus * 0x0012fb28) line 1060 PresShell::HandleEvent(PresShell * const 0x024e5234, nsIView * 0x024ff1a0, nsGUIEvent * 0x0012fc1c, nsEventStatus * 0x0012fb28) line 3019 + 39 bytes nsView::HandleEvent(nsView * const 0x024ff1a0, nsGUIEvent * 0x0012fc1c, unsigned int 8, nsEventStatus * 0x0012fb28, int & 0) line 799 nsView::HandleEvent(nsView * const 0x024f97b0, nsGUIEvent * 0x0012fc1c, unsigned int 8, nsEventStatus * 0x0012fb28, int & 0) line 784 nsView::HandleEvent(nsView * const 0x024f9850, nsGUIEvent * 0x0012fc1c, unsigned int 8, nsEventStatus * 0x0012fb28, int & 0) line 784 nsView::HandleEvent(nsView * const 0x024e5890, nsGUIEvent * 0x0012fc1c, unsigned int 28, nsEventStatus * 0x0012fb28, int & 0) line 784 nsViewManager2::DispatchEvent(nsViewManager2 * const 0x024e5b10, nsGUIEvent * 0x0012fc1c, nsEventStatus * 0x0012fb28) line 1216 HandleEvent(nsGUIEvent * 0x0012fc1c) line 69 nsWindow::DispatchEvent(nsWindow * const 0x024ffef4, nsGUIEvent * 0x0012fc1c, nsEventStatus & nsEventStatus_eIgnore) line 493 + 10 bytes nsWindow::DispatchWindowEvent(nsGUIEvent * 0x0012fc1c) line 514 nsWindow::DispatchMouseEvent(unsigned int 302, nsPoint * 0x00000000 {x=??? y=???}) line 2973 + 21 bytes ChildWindow::DispatchMouseEvent(unsigned int 302, nsPoint * 0x00000000 {x=??? y=???}) line 3191 nsWindow::ProcessMessage(unsigned int 513, unsigned int 1, long 1572907, long * 0x0012feb8) line 2254 + 24 bytes nsWindow::WindowProc(HWND__ * 0x09930826, unsigned int 513, unsigned int 1, long 1572907) line 684 + 27 bytes Here is the frame dump: webshell=01508190 Viewport(-1)@0117F7B0 [view=024E5890] {0,0,13380,2835} [state=00002014] sc=024F8 B90< Scroll(-1)@0117F828 [view=024F9850] {0,0,13380,2835} [state=00002004] sc=024F9 AA0< Root(-1)@0117F7EC [view=024FF1A0] {0,0,13380,2835} [state=00002000] sc=024FF 3A0< Area(html)(-1)@0117F864 {0,0,13380,2835} [state=000c0014] sc=024FECE0(i=1, b=1)< line 024FC940: count=1 state=inline,,,[0x400] {0,0,0,0} < Text(1)@0117F8B0[0,1,T] next=0117F8EC {0,0,0,0} [state=41000024] sc=0 24FE160< "\n" > > line 024FC8E0: count=1 state=block,,,[0x402] {120,120,13140,330} < Block(body)(2)@0117F8EC {120,120,13140,330} [state=00000014] sc=024FCB B0(i=1,b=1)< line 02506A90: count=3 state=inline,,,trimmed[0xc08] {0,0,1260,330} < Text(0)@0117F934[0,1,T] next=0117FAA4 {0,330,0,0} [state=41000024 ] sc=024FB480< "\n" > ComboboxControl(select)(1)@0117FAA4 next=0117FED8 {0,0,1260,330} [ state=00010034] sc=02504CE0(i=1,b=0)< line 02501E50: count=1 state=inline,,,[0x400] {30,30,240,270} < ButtonControl(input)(-1)@0117FD24 {990,30,240,270} [state=0000 04a4] sc=02501810< Area(input)(-1)@0117FDCC {75,195,0,0} [state=00090004] sc=02 503620(i=1,b=0)< line 02503460: count=1 state=inline,,,[0x400] {0,0,0,0} < Text(-1)@0117FE18[0,2,T] {0,0,0,0} [state=41000024] sc= 0155FC00< " " > > > > > Popup-list< Scroll(select)(1)@0117FB88 [view=02504080] {0,330,1200,300} [s tate=00002014] sc=0155E340< Area(select)(1)@0117FC54 [view=02502A40] {30,30,1140,240} [s tate=00012000] sc=0155F430(i=0,b=1)< line 02501EB0: count=1 state=block,,,[0x402] mew=840 {0,0, 1140,240} < Block(option)(0)@0117FCA0 {0,0,1140,240} [state=00000004 ] sc=0155C720(i=1,b=0)< line 02501FC0: count=1 state=inline,,,[0x400] mew=795 {45,0,795,240} < Text(0)@0117FCE8[0,8,T] {45,0,795,240} [state=40000 024] sc=0153A560< "One Fish" > > > > > > > > Text(2)@0117FED8[0,1,T] next=0117F970 {1260,105,0,285} [state=600 00024] sc=024FB480< "\n" > > line 02505C20: count=1 state=block,,,[0x402] {0,330,0,0} < TableOuter(table)(3)@0117F970 {0,330,0,0} [state=00000004] sc=024F A1F0< Table(table)(3)@0117F9C4 {0,0,0,0} [state=00000004] sc=024FA1F0< Block(form)(0)@0117FA30 {0,0,0,0} [state=00000004] sc=025002D0 (i=0,b=0)< > > > > > > > > > >

Added perf to keywords

I forgot to mention that you do not even have to select anything. This happens when the list is being dropped down and then again when it is popped up.

This is a performance issue, it causes dropsdowns to act slowly

Is this seen also my testcase to other bug? http://bugzilla.mozilla.org/showattachment.cgi?attach_id=6164 When you drop down leftmost combobox, its gets a bit wider and others shift right. This doesnt happen on first row, and only on leftmost column.

This is a duplicate of ye olde Bug 15155

*** This bug has been marked as a duplicate of 15155 ***

Verified duplicate.