Hide Reset to Default behind a hover-reveal corner marker

Reset to Default is a destructive-ish, rarely-used action that doesn't
need to compete for attention with the fields above it every time the
MinZ box is open. It now starts collapsed to a small "..." marker in
the box's bottom-right corner, and expands to the full button only
while the mouse is nearby (with a margin so the tiny marker isn't
fiddly to hit); moving away collapses it back.

The overlay draws as an absolute-position layer via SetCursorScreenPos
rather than in the normal top-to-bottom flow, and reserves no line of
its own - it sits on top of, and may partially overlap, the last
field's row instead of making the box taller.

Author: Evgeni Raikhel

common/embedded-filter-model.cpp

@@ -366,36 +366,74 @@ namespace rs2
         if( ImGui::IsItemHovered() )
             ImGui::SetTooltip( "Unchecked = Auto (firmware-computed threshold)" );
 
-        // Reset to Default - goes through the exact same touch()/finalize() pipeline as every
-        // other field edit above (one click = one discrete, debounced, atomically-committed
-        // change), rather than a separate commit path, so it gets the fade animation and the
-        // undo grace window for free. range.def is the FULL FW-reported default struct (header
-        // fields included); before_commit below forces enable back on regardless, same as any
-        // other edit in this box - "reset" doesn't leave the control disabled even if that's
-        // literally what the FW default says, for consistency with "touching this box means you
-        // want it on."
-        if( ImGui::Button( "Reset to Default##minz" ) )
+        ImGui::Dummy( ImVec2( 0, 2 ) );
+        float frame_bottom = ImGui::GetCursorScreenPos().y;
+        ImVec2 frame_min( frame_left, frame_top );
+        ImVec2 frame_max( frame_left + frame_width, frame_bottom );
+
+        // Reset to Default starts hidden behind a small "..." marker tucked into the box's
+        // bottom-right corner - this is a destructive-ish, rarely-used action that doesn't need
+        // to compete for attention with the fields above it every time the box is open. Hovering
+        // anywhere within reveal_margin of the corner (not just exactly on the tiny marker, which
+        // would be fiddly to hit) swaps it for the real button in the same spot; moving away
+        // collapses it back to "...". Drawn as an absolute-position overlay via
+        // SetCursorScreenPos rather than inline in the normal top-to-bottom flow, since its
+        // presence/absence shouldn't shift any of the fields above it - the cursor is restored
+        // afterward so the NEXT thing this panel draws isn't displaced. No line of its own is
+        // reserved for it (frame_bottom sits right after the last field's normal 2px pad), so
+        // the revealed button sits on top of - and may partially overlap - that last field's
+        // own row rather than pushing the box taller.
         {
-            try
+            ImVec2 saved_cursor = ImGui::GetCursorScreenPos();
+
+            ImVec2 button_size = ImGui::CalcTextSize( "Reset to Default" );
+            button_size.x += ImGui::GetStyle().FramePadding.x * 2.0f;
+            button_size.y += ImGui::GetStyle().FramePadding.y * 2.0f;
+            ImVec2 button_pos( frame_max.x - button_size.x - 4.0f, frame_max.y - button_size.y - 4.0f );
+
+            constexpr float reveal_margin = 24.0f;
+            ImVec2 mouse = ImGui::GetIO().MousePos;
+            bool nearby = mouse.x >= button_pos.x - reveal_margin && mouse.x <= frame_max.x + reveal_margin
+                       && mouse.y >= button_pos.y - reveal_margin && mouse.y <= frame_max.y + reveal_margin;
+
+            if( nearby )
             {
-                auto range = _embedded_filter->get_composite_option_range_as< rs2_minz_control_range >( id );
-                _minz_editor.value = range.def;
-                _minz_editor.touch();
-                _minz_editor.finalize();
+                // Goes through the exact same touch()/finalize() pipeline as every other field
+                // edit above (one click = one discrete, debounced, atomically-committed change),
+                // rather than a separate commit path, so it gets the fade animation and the undo
+                // grace window for free. range.def is the FULL FW-reported default struct (header
+                // fields included); before_commit below forces enable back on regardless, same as
+                // any other edit in this box - "reset" doesn't leave the control disabled even if
+                // that's literally what the FW default says, for consistency with "touching this
+                // box means you want it on."
+                ImGui::SetCursorScreenPos( button_pos );
+                if( ImGui::Button( "Reset to Default##minz" ) )
+                {
+                    try
+                    {
+                        auto range = _embedded_filter->get_composite_option_range_as< rs2_minz_control_range >( id );
+                        _minz_editor.value = range.def;
+                        _minz_editor.touch();
+                        _minz_editor.finalize();
+                    }
+                    catch( const std::exception & e )
+                    {
+                        error_message = e.what();
+                    }
+                }
+                any_field_active = any_field_active || ImGui::IsItemActive();
+                if( ImGui::IsItemHovered() )
+                    ImGui::SetTooltip( "Restore all fields to the firmware-reported default values" );
             }
-            catch( const std::exception & e )
+            else
             {
-                error_message = e.what();
+                ImVec2 marker_size = ImGui::CalcTextSize( "..." );
+                ImGui::SetCursorScreenPos( ImVec2( frame_max.x - marker_size.x - 8.0f, frame_max.y - marker_size.y - 6.0f ) );
+                ImGui::TextDisabled( "..." );
             }
-        }
-        any_field_active = any_field_active || ImGui::IsItemActive();
-        if( ImGui::IsItemHovered() )
-            ImGui::SetTooltip( "Restore all fields to the firmware-reported default values" );
 
-        ImGui::Dummy( ImVec2( 0, 2 ) );
-        float frame_bottom = ImGui::GetCursorScreenPos().y;
-        ImVec2 frame_min( frame_left, frame_top );
-        ImVec2 frame_max( frame_left + frame_width, frame_bottom );
+            ImGui::SetCursorScreenPos( saved_cursor );
+        }
 
         // Draws the dirty-state fade/border, the hover tooltip, and sends the whole struct once
         // the debounce timer lapses (or focus leaves the group early) - see