Selectable: Clarifying the code around use of ImGuiSelectableFlags_DrawFillAvailWidth (with intent of trying to remove it).

Amend old 6251d379, 2bcafc86

imgui.cpp

@@ -6818,6 +6818,8 @@ static void ImGui::ErrorCheckBeginEndCompareStacksSize(ImGuiWindow* window, bool
 //-----------------------------------------------------------------------------
 
 // Advance cursor given item size for layout.
+// Register minimum needed size so it can extend the bounding box used for auto-fit calculation.
+// See comments in ItemAdd() about how/why the size provided to ItemSize() vs ItemAdd() may often different.
 void ImGui::ItemSize(const ImVec2& size, float text_baseline_y)
 {
     ImGuiContext& g = *GImGui;
@@ -6858,7 +6860,7 @@ void ImGui::ItemSize(const ImRect& bb, float text_baseline_y)
 
 // Declare item bounding box for clipping and interaction.
 // Note that the size can be different than the one provided to ItemSize(). Typically, widgets that spread over available surface
-// declare their minimum size requirement to ItemSize() and then use a larger region for drawing/interaction, which is passed to ItemAdd().
+// declare their minimum size requirement to ItemSize() and provide a larger region to ItemAdd() which is used drawing/interaction.
 bool ImGui::ItemAdd(const ImRect& bb, ImGuiID id, const ImRect* nav_bb_arg)
 {
     ImGuiContext& g = *GImGui;

imgui_internal.h

@@ -494,7 +494,7 @@ enum ImGuiSelectableFlagsPrivate_
     ImGuiSelectableFlags_NoHoldingActiveID  = 1 << 20,
     ImGuiSelectableFlags_SelectOnClick      = 1 << 21,  // Override button behavior to react on Click (default is Click+Release)
     ImGuiSelectableFlags_SelectOnRelease    = 1 << 22,  // Override button behavior to react on Release (default is Click+Release)
-    ImGuiSelectableFlags_DrawFillAvailWidth = 1 << 23,  // FIXME: We may be able to remove this (added in 6251d379 for menus)
+    ImGuiSelectableFlags_DrawFillAvailWidth = 1 << 23,  // Draw over all avail width even if we declared less for layout. FIXME: We may be able to remove this (added in 6251d379, 2bcafc86 for menus)
     ImGuiSelectableFlags_DrawHoveredWhenHeld= 1 << 24,  // Always show active when held, even is not hovered. This concept could probably be renamed/formalized somehow.
     ImGuiSelectableFlags_SetNavIdOnHover    = 1 << 25
 };

imgui_widgets.cpp

@@ -5578,15 +5578,11 @@ bool ImGui::Selectable(const char* label, bool selected, ImGuiSelectableFlags fl
     ImRect bb_inner(pos, pos + size);
     ItemSize(size, 0.0f);
 
-    // Fill horizontal space.
-    ImVec2 window_padding = window->WindowPadding;
-    float max_x = (flags & ImGuiSelectableFlags_SpanAllColumns) ? GetWindowContentRegionMax().x + window->Pos.x : GetContentRegionMaxAbs().x;
-    float w_draw = ImMax(label_size.x, max_x - window_padding.x - pos.x);
-    ImVec2 size_draw((size_arg.x != 0 && !(flags & ImGuiSelectableFlags_DrawFillAvailWidth)) ? size_arg.x : w_draw, size.y);
-
-    ImRect bb_align(pos, pos + size_draw);
+    // Fill horizontal space
+    const float max_x = (flags & ImGuiSelectableFlags_SpanAllColumns) ? GetWindowContentRegionMax().x + window->Pos.x : GetContentRegionMaxAbs().x;
     if (size_arg.x == 0.0f || (flags & ImGuiSelectableFlags_DrawFillAvailWidth))
-        bb_align.Max.x += window_padding.x;
+        size.x = ImMax(label_size.x, max_x - window->WindowPadding.x - pos.x) + window->WindowPadding.x;
+    ImRect bb_align(pos, pos + size);
 
     // Selectables are meant to be tightly packed together with no click-gap, so we extend the box to cover spacing between selectable.
     ImRect bb_enlarged = bb_align;
@@ -6228,10 +6224,12 @@ bool ImGui::BeginMenu(const char* label, bool enabled)
     else
     {
         // Menu inside a menu
+        // (In a typical menu window where all items are BeginMenu() or MenuItem() calls, extra_w will always be 0.0f.
+        //  Only when they are other items sticking out we're going to add spacing, yet only register minimum width into the layout system.
         popup_pos = ImVec2(pos.x, pos.y - style.WindowPadding.y);
-        float w = window->DC.MenuColumns.DeclColumns(label_size.x, 0.0f, IM_FLOOR(g.FontSize * 1.20f)); // Feedback to next frame
-        float extra_w = ImMax(0.0f, GetContentRegionAvail().x - w);
-        pressed = Selectable(label, menu_is_open, ImGuiSelectableFlags_NoHoldingActiveID | ImGuiSelectableFlags_SelectOnClick | ImGuiSelectableFlags_DontClosePopups | ImGuiSelectableFlags_DrawFillAvailWidth | (!enabled ? ImGuiSelectableFlags_Disabled : 0), ImVec2(w, 0.0f));
+        float min_w = window->DC.MenuColumns.DeclColumns(label_size.x, 0.0f, IM_FLOOR(g.FontSize * 1.20f)); // Feedback to next frame
+        float extra_w = ImMax(0.0f, GetContentRegionAvail().x - min_w);
+        pressed = Selectable(label, menu_is_open, ImGuiSelectableFlags_NoHoldingActiveID | ImGuiSelectableFlags_SelectOnClick | ImGuiSelectableFlags_DontClosePopups | ImGuiSelectableFlags_DrawFillAvailWidth | (!enabled ? ImGuiSelectableFlags_Disabled : 0), ImVec2(min_w, 0.0f));
         ImU32 text_col = GetColorU32(enabled ? ImGuiCol_Text : ImGuiCol_TextDisabled);
         RenderArrow(window->DrawList, pos + ImVec2(window->DC.MenuColumns.Pos[2] + extra_w + g.FontSize * 0.30f, 0.0f), text_col, ImGuiDir_Right);
     }
@@ -6376,11 +6374,14 @@ bool ImGui::MenuItem(const char* label, const char* shortcut, bool selected, boo
     }
     else
     {
-        ImVec2 shortcut_size = shortcut ? CalcTextSize(shortcut, NULL) : ImVec2(0.0f, 0.0f);
-        float w = window->DC.MenuColumns.DeclColumns(label_size.x, shortcut_size.x, IM_FLOOR(g.FontSize * 1.20f)); // Feedback for next frame
-        float extra_w = ImMax(0.0f, GetContentRegionAvail().x - w);
-        pressed = Selectable(label, false, flags | ImGuiSelectableFlags_DrawFillAvailWidth, ImVec2(w, 0.0f));
-        if (shortcut_size.x > 0.0f)
+        // Menu item inside a vertical menu
+        // (In a typical menu window where all items are BeginMenu() or MenuItem() calls, extra_w will always be 0.0f.
+        //  Only when they are other items sticking out we're going to add spacing, yet only register minimum width into the layout system.
+        float shortcut_w = shortcut ? CalcTextSize(shortcut, NULL).x : 0.0f;
+        float min_w = window->DC.MenuColumns.DeclColumns(label_size.x, shortcut_w, IM_FLOOR(g.FontSize * 1.20f)); // Feedback for next frame
+        float extra_w = ImMax(0.0f, GetContentRegionAvail().x - min_w);
+        pressed = Selectable(label, false, flags | ImGuiSelectableFlags_DrawFillAvailWidth, ImVec2(min_w, 0.0f));
+        if (shortcut_w > 0.0f)
         {
             PushStyleColor(ImGuiCol_Text, g.Style.Colors[ImGuiCol_TextDisabled]);
             RenderText(pos + ImVec2(window->DC.MenuColumns.Pos[1] + extra_w, 0.0f), shortcut, NULL, false);